diff --git a/check/fixes.frm b/check/fixes.frm
index 3ad37168..d16b8523 100644
--- a/check/fixes.frm
+++ b/check/fixes.frm
@@ -1409,6 +1409,63 @@ assert stdout =~ exact_pattern(<<'EOF')
    F
 EOF
 *--#] Issue129_2 :
+*--#[ Issue138_1 :
+CFunction f,a,b,c,d;
+Symbol x,y;
+Local F = (1+f((a+b)^2,(a+b)*(c*d)))^20;
+.sort
+Identify f(x?,y?) = f(gcd_(x,y));
+ToPolynomial;
+.sort
+FromPolynomial;
+Identify f(a+b) = -1;
+Print;
+.end
+assert succeeded?
+assert result("F") =~ expr("0")
+*--#] Issue138_1 :
+*--#[ Issue138_2 :
+CFunction f,g;
+Symbol x,y;
+Local F = (1+f((g(x)+g(y))^2))^20;
+FactArg f;
+ToPolynomial;
+.sort
+FromPolynomial;
+Identify f(g(x)+g(y),g(x)+g(y)) = -1;
+Print;
+.end
+assert succeeded?
+assert result("F") =~ expr("0")
+*--#] Issue138_2 :
+*--#[ Issue138_3 :
+CFunction f,g;
+Symbol x,y;
+Local F = (1+f((g(x)+g(y))^2))^20;
+FactArg f;
+ArgToExtraSymbol g;
+.sort
+FromPolynomial;
+Identify f(g(x)+g(y),g(x)+g(y)) = -1;
+Print;
+.end
+assert succeeded?
+assert result("F") =~ expr("0")
+*--#] Issue138_3 :
+*--#[ Issue138_4 :
+CFunction f,g,h;
+Symbol x,y;
+Local F = (1+f((g(x)+g(y)+h(x)+h(y))^2))^20;
+ArgToExtraSymbol g;
+FactArg f;
+.sort
+FromPolynomial;
+Identify f(g(x)+g(y)+h(x)+h(y),g(x)+g(y)+h(x)+h(y)) = -1;
+Print;
+.end
+assert succeeded?
+assert result("F") =~ expr("0")
+*--#] Issue138_4 :
 *--#[ Issue139 :
 * Corrupted characters in printing f(-2147483648)
 CF f;
diff --git a/sources/argument.c b/sources/argument.c
index 12a2308f..069d37f5 100644
--- a/sources/argument.c
+++ b/sources/argument.c
@@ -2076,7 +2076,7 @@ int ArgFactorize(PHEAD WORD *argin, WORD *argout)
 #ifdef NEWORDER
 	WORD *tt;
 #endif
-	WORD startebuf = cbuf[AT.ebufnum].numrhs,oldword;
+	WORD startebuf = cbuf[AT.ebufnum].numrhs, extrasymbolbase = -1, oldword;
 	WORD oldsorttype = AR.SortType, numargs;
 	int error = 0, action = 0, i, ii, number, sign = 1;
 
@@ -2254,7 +2254,7 @@ int ArgFactorize(PHEAD WORD *argin, WORD *argout)
 		argextra = AT.WorkPointer;
 		NewSort(BHEAD0);
 		while ( t < tstop ) {
-			if ( LocalConvertToPoly(BHEAD t,argextra,startebuf,0) < 0 ) {
+			if ( LocalConvertToPoly(BHEAD t,argextra,startebuf,0,&extrasymbolbase) < 0 ) {
 				error = -1;
 getout:
 				AR.SortType = oldsorttype;
@@ -2309,8 +2309,8 @@ int ArgFactorize(PHEAD WORD *argin, WORD *argout)
 				argextra = AT.WorkPointer;
 				NewSort(BHEAD0);
 				while ( t < tstop ) {
-					if ( ConvertFromPoly(BHEAD t,argextra,numxsymbol,CC->numrhs-startebuf+numxsymbol
-					,startebuf-numxsymbol,1) <= 0 ) {
+					if ( ConvertFromPoly(BHEAD t,argextra,extrasymbolbase,CC->numrhs-startebuf+extrasymbolbase
+					,startebuf-extrasymbolbase,1) <= 0 ) {
 						TermFree(argcopy2,"argcopy2");
 						LowerSortLevel();
 						error = -3;
diff --git a/sources/compcomm.c b/sources/compcomm.c
index 4c281613..55a11766 100644
--- a/sources/compcomm.c
+++ b/sources/compcomm.c
@@ -2223,11 +2223,6 @@ int CoSplitLastArg(UBYTE *s) { return(DoArgument(s,TYPESPLITLASTARG)); }
 */
 
 int CoFactArg(UBYTE *s) {
-	if ( ( AC.topolynomialflag & TOPOLYNOMIALFLAG ) != 0 ) {
-		MesPrint("&ToPolynomial statement and FactArg statement are not allowed in the same module");
-		return(1);
-	}
-	AC.topolynomialflag |= FACTARGFLAG;
 	return(DoArgument(s,TYPEFACTARG));
 }
 
@@ -5953,9 +5948,8 @@ int CoDropSymbols(UBYTE *s)
 
 	Converts the current term as much as possible to symbols.
 	Keeps a list of all objects converted to symbols in AM.sbufnum.
-	Note that this cannot be executed in parallel because we have only
-	a single compiler buffer for this. Hence we switch on the noparallel
-	module option.
+	There is only a single compiler buffer for this. In TFORM we protect access
+	to it with AM.sbuflock; in ParFORM the statement is executed on the master.
 
 	Option(s):
 		OnlyFunctions [,name1][,name2][,...,namem];
@@ -5965,10 +5959,6 @@ int CoToPolynomial(UBYTE *inp)
 {
 	int error = 0;
 	while ( *inp == ' ' || *inp == ',' || *inp == '\t' ) inp++;
-	if ( ( AC.topolynomialflag & ~TOPOLYNOMIALFLAG ) != 0 ) {
-		MesPrint("&ToPolynomial statement and FactArg statement are not allowed in the same module");
-		return(1);
-	}
 	if ( AO.OptimizeResult.code != NULL ) {
 		MesPrint("&Using ToPolynomial statement when there are still optimization results active.");
 		MesPrint("&Please use #ClearOptimize instruction first.");
@@ -6067,11 +6057,6 @@ int CoArgToExtraSymbol(UBYTE *s)
 	CBUF *C = cbuf + AC.cbufnum;
 	WORD *lhs;
 
-	/* TODO: resolve interference with rational arithmetic. (#138) */
-	if ( ( AC.topolynomialflag & ~TOPOLYNOMIALFLAG ) != 0 ) {
-		MesPrint("&ArgToExtraSymbol statement and FactArg statement are not allowed in the same module");
-		return(1);
-	}
 	if ( AO.OptimizeResult.code != NULL ) {
 		MesPrint("&Using ArgToExtraSymbol statement when there are still optimization results active.");
 		MesPrint("&Please use #ClearOptimize instruction first.");
diff --git a/sources/comtool.c b/sources/comtool.c
index 06ee9263..a54f4133 100644
--- a/sources/comtool.c
+++ b/sources/comtool.c
@@ -526,6 +526,8 @@ balance:;
 	Returns -1 if the element is not in the tree.
 	The advantage of this routine over InsTree is that this routine
 	can be run in parallel.
+	Note that this function is not actually thread-safe, due to the update
+	of "usage".
 */
 
 int FindTree(int bufnum, WORD *subexpr)
diff --git a/sources/declare.h b/sources/declare.h
index cd4fb2e4..a8d62dfb 100644
--- a/sources/declare.h
+++ b/sources/declare.h
@@ -1576,10 +1576,10 @@ extern int RunHtoZArg(PHEAD WORD *fun, WORD *args);
 extern int NormPolyTerm(PHEAD WORD *);
 extern WORD ComparePoly(WORD *, WORD *, WORD);
 extern int ConvertToPoly(PHEAD WORD *, WORD *,WORD *,WORD);
-extern int LocalConvertToPoly(PHEAD WORD *, WORD *, WORD,WORD);
+extern int LocalConvertToPoly(PHEAD WORD *, WORD *, WORD,WORD,WORD *);
 extern int ConvertFromPoly(PHEAD WORD *, WORD *, WORD, WORD, WORD, WORD);
 extern int FindSubterm(WORD *);
-extern int FindLocalSubterm(PHEAD WORD *, WORD);
+extern int FindLocalSubterm(PHEAD WORD *, WORD, WORD);
 extern void PrintSubtermList(int,int);
 extern void PrintExtraSymbol(int,WORD *,int);
 extern int FindSubexpression(WORD *);
@@ -1606,8 +1606,8 @@ extern WORD *PolyDiv(PHEAD WORD *,WORD *,char *);
 extern void GCDclean(PHEAD WORD *, WORD *);
 extern WORD *TakeSymbolContent(PHEAD WORD *,WORD *);
 extern int GCDterms(PHEAD WORD *,WORD *,WORD *);
-extern WORD *PutExtraSymbols(PHEAD WORD *,WORD,int *);
-extern WORD *TakeExtraSymbols(PHEAD WORD *,WORD);
+extern WORD *PutExtraSymbols(PHEAD WORD *,WORD,int *,WORD *);
+extern WORD *TakeExtraSymbols(PHEAD WORD *,WORD,WORD);
 extern WORD *MultiplyWithTerm(PHEAD WORD *, WORD *,WORD);
 extern WORD *TakeContent(PHEAD WORD *, WORD *);
 extern int MergeSymbolLists(PHEAD WORD *, WORD *, int);
diff --git a/sources/dollar.c b/sources/dollar.c
index 0b50ff52..0d323808 100644
--- a/sources/dollar.c
+++ b/sources/dollar.c
@@ -2949,7 +2949,7 @@ int DollarFactorize(PHEAD WORD numdollar)
 #endif
 	int i, j, jj, action = 0, sign = 1;
 	LONG insize, ii;
-	WORD startebuf = cbuf[AT.ebufnum].numrhs;
+	WORD startebuf = cbuf[AT.ebufnum].numrhs, extrasymbolbase = -1;
 	WORD nfactors, factorsincontent, extrafactor = 0;
 	WORD oldsorttype = AR.SortType;
 
@@ -3144,7 +3144,7 @@ int DollarFactorize(PHEAD WORD numdollar)
 		NewSort(BHEAD0);
 		NewSort(BHEAD0);
 		while ( *t ) {
-			if ( LocalConvertToPoly(BHEAD t,termextra,startebuf,0) < 0 ) {
+			if ( LocalConvertToPoly(BHEAD t,termextra,startebuf,0,&extrasymbolbase) < 0 ) {
 getout:
 				AR.SortType = oldsorttype;
 				M_free(buf1,"DollarFactorize-2");
@@ -3274,8 +3274,8 @@ int DollarFactorize(PHEAD WORD numdollar)
 			NewSort(BHEAD0);
 			NewSort(BHEAD0);
 			while ( *term ) {
-				if ( ConvertFromPoly(BHEAD term,argextra,numxsymbol,CC->numrhs-startebuf+numxsymbol
-				,startebuf-numxsymbol,1) <= 0 ) {
+				if ( ConvertFromPoly(BHEAD term,argextra,extrasymbolbase,CC->numrhs-startebuf+extrasymbolbase
+				,startebuf-extrasymbolbase,1) <= 0 ) {
 					LowerSortLevel();
 getout2:			AR.SortType = oldsorttype;
 					M_free(d->factors,"factors in dollar");
diff --git a/sources/ftypes.h b/sources/ftypes.h
index 999a9014..091accb7 100644
--- a/sources/ftypes.h
+++ b/sources/ftypes.h
@@ -1043,7 +1043,6 @@ typedef int (*TFUN1)(UBYTE *,int);
 #define NOLYNDON 2
 
 #define TOPOLYNOMIALFLAG 1
-#define FACTARGFLAG 2
 
 #define OLDFACTARG 1
 #define NEWFACTARG 0
diff --git a/sources/notation.c b/sources/notation.c
index 64107ad8..6f7c2d19 100644
--- a/sources/notation.c
+++ b/sources/notation.c
@@ -505,13 +505,25 @@ int ConvertToPoly(PHEAD WORD *term, WORD *outterm, WORD *comlist, WORD par)
  *		buffer and represented by a symbol. The numbering of the extra
  *		symbols is down from the maximum. In principle there can be a
  *		problem when running into the already assigned ones.
- *		This uses the FindTree for searching in the global tree and
- *		then looks further in the AT.ebufnum. This allows fully parallel
- *		processing. Hence we need no locks. Cannot be used in the same
- *		module as ConvertToPoly.
+ *		The temporary extrasymbol definitions live in the private AT.ebufnum.
+ *		We don't touch the global definitions, beyond determining the current
+ *		number of global extrasymbols, under a lock.
  */
 
-int LocalConvertToPoly(PHEAD WORD *term, WORD *outterm, WORD startebuf, WORD par)
+/** Helper function: copy the current numxsymbol, under a lock. Local temporary
+ * extrasymbol numbers will follow this. If the argument is positive, we have
+ * already stored the global numxsymbol and don't store it again. */
+static WORD LocalExtraSymbolBase(WORD *base)
+{
+	if ( *base < 0 ) {
+		LOCK(AM.sbuflock);
+		*base = numxsymbol;
+		UNLOCK(AM.sbuflock);
+	}
+	return(*base);
+}
+
+int LocalConvertToPoly(PHEAD WORD *term, WORD *outterm, WORD startebuf, WORD par, WORD *base)
 {
 	WORD *tout, *tstop, ncoef, *t, *r, *tt, *ttwo = 0;
 	int i, action = 0;
@@ -536,7 +548,7 @@ int LocalConvertToPoly(PHEAD WORD *term, WORD *outterm, WORD startebuf, WORD par
 					tout[2] = 4;
 					tout[3] = r[0];
 					tout[4] = -1;
-					i = FindLocalSubterm(BHEAD tout+1,startebuf);
+					i = FindLocalSubterm(BHEAD tout+1,startebuf,LocalExtraSymbolBase(base));
 					*tout++ = SYMBOL;
 					*tout++ = 4;
 					*tout++ = MAXVARIABLES-i;
@@ -560,7 +572,7 @@ int LocalConvertToPoly(PHEAD WORD *term, WORD *outterm, WORD startebuf, WORD par
 				else {
 					tout[5] = 1;
 				}
-				i = FindLocalSubterm(BHEAD tout+1,startebuf);
+				i = FindLocalSubterm(BHEAD tout+1,startebuf,LocalExtraSymbolBase(base));
 				*tout++ = SYMBOL;
 				*tout++ = 4;
 				*tout++ = MAXVARIABLES-i;
@@ -577,7 +589,7 @@ int LocalConvertToPoly(PHEAD WORD *term, WORD *outterm, WORD startebuf, WORD par
 				tout[2] = 4;
 				tout[3] = r[0];
 				tout[4] = r[1];
-				i = FindLocalSubterm(BHEAD tout+1,startebuf);
+				i = FindLocalSubterm(BHEAD tout+1,startebuf,LocalExtraSymbolBase(base));
 				*tout++ = SYMBOL;
 				*tout++ = 4;
 				*tout++ = MAXVARIABLES-i;
@@ -593,7 +605,7 @@ int LocalConvertToPoly(PHEAD WORD *term, WORD *outterm, WORD startebuf, WORD par
 				tout[1] = INDEX;
 				tout[2] = 3;
 				tout[3] = r[0];
-				i = FindLocalSubterm(BHEAD tout+1,startebuf);
+				i = FindLocalSubterm(BHEAD tout+1,startebuf,LocalExtraSymbolBase(base));
 				*tout++ = SYMBOL;
 				*tout++ = 4;
 				*tout++ = MAXVARIABLES-i;
@@ -615,7 +627,7 @@ int LocalConvertToPoly(PHEAD WORD *term, WORD *outterm, WORD startebuf, WORD par
 			else { t += t[1]; }
 		}
 		else if ( *t >= FUNCTION ) {
-			i = FindLocalSubterm(BHEAD t,startebuf);
+			i = FindLocalSubterm(BHEAD t,startebuf,LocalExtraSymbolBase(base));
 			t += t[1];
 			*tout++ = SYMBOL;
 			*tout++ = 4;
@@ -661,65 +673,61 @@ int LocalConvertToPoly(PHEAD WORD *term, WORD *outterm, WORD startebuf, WORD par
 		subexpressions when extra symbols have been replaced.
 */
 
+#ifdef WITHPTHREADS
+/** Helper function: we only lock sbuflock if the term actually has an
+ * extrasymbol to replace. Search for one. */
+static int PolyHasExtraSymbols(WORD *term, WORD from, WORD to)
+{
+	WORD *t, *tt, *tstop, *tstop1;
+
+	tstop = term + *term - ABS(term[*term-1]);
+	for ( t = term + 1; t < tstop; t += t[1] ) {
+		if ( *t != SYMBOL ) {
+			continue;
+		}
+		tstop1 = t + t[1];
+		for ( tt = t + 2; tt < tstop1; tt += 2 ) {
+			if ( *tt >= MAXVARIABLES - to && *tt < MAXVARIABLES - from ) {
+				return(1);
+			}
+		}
+	}
+	return(0);
+}
+#endif
+
+static int ConvertFromPolyImpl(PHEAD WORD *term, WORD *outterm, WORD from, WORD to, WORD offset, WORD par);
+
 int ConvertFromPoly(PHEAD WORD *term, WORD *outterm, WORD from, WORD to, WORD offset, WORD par)
+{
+	int locked = 0, result;
+	/* LocalConvertToPoly uses only AT.ebufnum. Expression-level FromPolynomial
+	 * needs the global lock if the term contains a global extrasymbol. */
+#ifdef WITHPTHREADS
+	if ( ! par && PolyHasExtraSymbols(term,from,to) ) {
+		LOCK(AM.sbuflock);
+		locked = 1;
+	}
+#endif
+	result = ConvertFromPolyImpl(BHEAD term,outterm,from,to,offset,par);
+#ifdef WITHPTHREADS
+	if ( locked ) {
+		UNLOCK(AM.sbuflock);
+	}
+#else
+	DUMMYUSE(locked);
+#endif
+	return(result);
+}
+
+static int ConvertFromPolyImpl(PHEAD WORD *term, WORD *outterm, WORD from, WORD to, WORD offset, WORD par)
 {
 	WORD *tout, *tstop, *tstop1, ncoef, *t, *r, *tt;
 	int i;
-/*	first = 1; */
 	tt = term + *term;
 	tout = outterm+1;
 	ncoef = ABS(tt[-1]);
 	tstop = tt - ncoef;
-/*
-	r = t = term + 1;
-	while ( t < tstop ) {
-		if ( *t == SYMBOL ) {
-			tstop1 = t + t[1];
-			tt = t + 2;
-			while ( tt < tstop1 ) {
-				if ( ( *tt < MAXVARIABLES - to )
-				  || ( *tt >= MAXVARIABLES - from ) ) {
-					tt += 2;
-				}
-				else break;
-			}
-			if ( tt >= tstop1 ) { t = tstop1; continue; }
-			while ( r < t ) *tout++ = *r++;
-			t += 2;
-			first = 0;
-			while ( t < tstop1 ) {
-				if ( ( *t < MAXVARIABLES - to )
-				  || ( *t >= MAXVARIABLES - from ) ) {
-					*tout++ = SYMBOL;
-					*tout++ = 4;
-					*tout++ = *t++;
-					*tout++ = *t++;
-				}
-				else {
-					*tout++ = SUBEXPRESSION;
-					*tout++ = SUBEXPSIZE;
-					*tout++ = MAXVARIABLES - *t++ + offset;
-					*tout++ = *t++;
-					if ( par ) *tout++ = AT.ebufnum;
-					else       *tout++ = AM.sbufnum;
-					FILLSUB(tout)
-				}
-			}
-			r = t;
-		}
-		else {
-			t += t[1];
-		}
-	}
-	if ( first ) {
-		i = *term; t = term;
-		NCOPY(outterm,t,i);
-		return(*term);
-	}
-	while ( r < t ) *tout++ = *r++;
-	NCOPY(tout,tstop,ncoef)
-	*outterm = tout-outterm;
-*/
 	t = term + 1;
 	while ( t < tstop ) {
 		if ( *t == SYMBOL ) {
@@ -771,7 +779,8 @@ int ConvertFromPoly(PHEAD WORD *term, WORD *outterm, WORD from, WORD to, WORD of
 		Searching is by tree structure.
 		Adding changes the tree.
 
-		Notice that in TFORM we should be in sequential mode.
+		Writing to the global extra-symbol buffer and its search tree is protected
+		in TFORM by AM.sbuflock.
 */
 
 int FindSubterm(WORD *subterm)
@@ -837,14 +846,15 @@ int FindSubterm(WORD *subterm)
  		#[ FindLocalSubterm :
 
 		In this routine we look up a variable.
-		If we don't find it we will enter it in the subterm compiler buffer
-		Searching is by tree structure.
-		Adding changes the tree.
+		If we don't find it we enter it in the thread-private compiler buffer.
 
-		Notice that in TFORM we should be in sequential mode.
+		The definitions and lookup are private to this thread. The numbering
+		starts after the stored global base, but may overlap global symbols
+		created by other threads after that store. The definitions are temporary,
+		so this doesn't matter.
 */
 
-int FindLocalSubterm(PHEAD WORD *subterm, WORD startebuf)
+int FindLocalSubterm(PHEAD WORD *subterm, WORD startebuf, WORD base)
 {
 	WORD old[5], *ss, *term, i, j, *t1, *t2;
 	int number;
@@ -857,13 +867,7 @@ int FindLocalSubterm(PHEAD WORD *subterm, WORD startebuf)
 	old[0] = *term; old[1] = ss[0]; old[2] = ss[1]; old[3] = ss[2]; old[4] = ss[3];
 	ss[0] = 1; ss[1] = 1; ss[2] = 3; ss[3] = 0; *term = subterm[1]+4;
 /*
-		First see whether we have this one already in the global buffer.
-*/
-	number = FindTree(AM.sbufnum,term);
-	if ( number > 0 ) goto wearehappy;
-/*
-	Now look whether it is in the ebufnum between startebuf and numrhs
-	Note however that we need an offset of (numxsymbol-startebuf)
+	Look whether it is in the private ebufnum between startebuf and numrhs.
 */
 	for ( i = startebuf+1; i <= C->numrhs; i++ ) {
 		t1 = C->rhs[i]; t2 = term;
@@ -871,7 +875,7 @@ int FindLocalSubterm(PHEAD WORD *subterm, WORD startebuf)
 			j = *t1;
 			while ( *t1 == *t2 && j > 0 ) { t1++; t2++; j--; }
 			if ( j <= 0 ) {
-				number = i-startebuf+numxsymbol;
+				number = i-startebuf+base;
 				goto wearehappy;
 			}
 		}
@@ -882,7 +886,7 @@ int FindLocalSubterm(PHEAD WORD *subterm, WORD startebuf)
 	AddRHS(AT.ebufnum,1);
 	AddNtoC(AT.ebufnum,*term,term,9);
 	AddToCB(C,0)
-	number = C->numrhs-startebuf+numxsymbol;
+	number = C->numrhs-startebuf+base;
 wearehappy:
 	*term = old[0]; ss[0] = old[1]; ss[1] = old[2]; ss[2] = old[3]; ss[3] = old[4];
 	return(number);
@@ -1094,7 +1098,8 @@ void PrintExtraSymbol(int num, WORD *terms,int par)
 		Searching is by tree structure.
 		Adding changes the tree.
 
-		Notice that in TFORM we should be in sequential mode.
+		Writing to the global extra-symbol buffer and its search tree is protected
+		in TFORM by AM.sbuflock.
 */
 
 int FindSubexpression(WORD *subexpr)
diff --git a/sources/polywrap.cc b/sources/polywrap.cc
index db085107..072daa2c 100644
--- a/sources/polywrap.cc
+++ b/sources/polywrap.cc
@@ -1194,6 +1194,7 @@ int poly_factorize_expression(EXPRESSIONS expr) {
 	
 	WORD *term = AT.WorkPointer;
 	WORD startebuf = cbuf[AT.ebufnum].numrhs;
+	WORD extrasymbolbase = -1;
 	FILEHANDLE *file;
 	POSITION pos;
 
@@ -1275,7 +1276,7 @@ int poly_factorize_expression(EXPRESSIONS expr) {
 			Terminate(-1);
 		}
 		buffer.check_memory(bufpos);		
-		if (LocalConvertToPoly(BHEAD term, buffer.terms + bufpos, startebuf,0) < 0) {
+		if (LocalConvertToPoly(BHEAD term, buffer.terms + bufpos,startebuf,0,&extrasymbolbase) < 0) {
 /* INTERNAL_ERROR_EXCL_START */
 			MesPrint("!>ERROR: in LocalConvertToPoly [factorize_expression]");
 			Terminate(-1);
@@ -1413,8 +1414,8 @@ int poly_factorize_expression(EXPRESSIONS expr) {
 					
 					for (WORD *t=buffer.terms; *t!=0; t+=*t) {
 						// substitute extra symbols
-						if (ConvertFromPoly(BHEAD t, term, numxsymbol, CC->numrhs-startebuf+numxsymbol,
-																startebuf-numxsymbol, 1) <= 0 ) {
+						if (ConvertFromPoly(BHEAD t, term, extrasymbolbase, CC->numrhs-startebuf+extrasymbolbase,
+												startebuf-extrasymbolbase, 1) <= 0 ) {
 /* INTERNAL_ERROR_EXCL_START */
 							MesPrint("!>ERROR: in ConvertFromPoly [factorize_expression]");
 							Terminate(-1);
@@ -1430,7 +1431,9 @@ int poly_factorize_expression(EXPRESSIONS expr) {
 
 					// sort and store in buffer
 					WORD *buffer;
-					if (EndSort(BHEAD (WORD *)((void *)(&buffer)),2) < 0) return -1;
+					if (EndSort(BHEAD (WORD *)((void *)(&buffer)),2) < 0) {
+						return -1;
+					}
 					
 					LONG bufsize=0;
 					for (WORD *t=buffer; *t!=0; t+=*t)
diff --git a/sources/ratio.c b/sources/ratio.c
index e1f29daa..6849d3f8 100644
--- a/sources/ratio.c
+++ b/sources/ratio.c
@@ -1150,7 +1150,7 @@ WORD *GCDfunction3(PHEAD WORD *in1, WORD *in2)
 	WORD oldsorttype = AR.SortType, *ow = AT.WorkPointer;;
 	WORD *t, *tt, *gcdout, *term1, *term2, *confree1, *confree2, *gcdout1, *proper1, *proper2;
 	int i, actionflag1, actionflag2;
-	WORD startebuf = cbuf[AT.ebufnum].numrhs;
+	WORD startebuf = cbuf[AT.ebufnum].numrhs, extrasymbolbase = -1;
 	WORD tryterm1, tryterm2;
 	if ( in2[*in2] == 0 ) { t = in1; in1 = in2; in2 = t; }
 	if ( in1[*in1] == 0 ) {	/* First input with only one term */
@@ -1187,7 +1187,7 @@ WORD *GCDfunction3(PHEAD WORD *in1, WORD *in2)
 	Now we have to replace all non-symbols and symbols to a negative power
 	by extra symbols.
 */
-	if ( ( proper1 = PutExtraSymbols(BHEAD confree1,startebuf,&actionflag1) ) == 0 ) goto CalledFrom;
+	if ( ( proper1 = PutExtraSymbols(BHEAD confree1,startebuf,&actionflag1,&extrasymbolbase) ) == 0 ) goto CalledFrom;
 	if ( confree1 != in1 ) {
 		if ( tryterm1 ) { TermFree(confree1,"TakeContent"); }
 		else { M_free(confree1,"TakeContent"); }
@@ -1195,7 +1195,7 @@ WORD *GCDfunction3(PHEAD WORD *in1, WORD *in2)
 /*
 	TermFree(confree1,"TakeSymbolContent");
 */
-	if ( ( proper2 = PutExtraSymbols(BHEAD confree2,startebuf,&actionflag2) ) == 0 ) goto CalledFrom;
+	if ( ( proper2 = PutExtraSymbols(BHEAD confree2,startebuf,&actionflag2,&extrasymbolbase) ) == 0 ) goto CalledFrom;
 	if ( confree2 != in2 ) {
 		if ( tryterm2 ) { TermFree(confree2,"TakeContent"); }
 		else { M_free(confree2,"TakeContent"); }
@@ -1212,7 +1212,7 @@ WORD *GCDfunction3(PHEAD WORD *in1, WORD *in2)
 
 	AR.SortType = oldsorttype;
 	if ( actionflag1 || actionflag2 ) {
-		if ( ( gcdout = TakeExtraSymbols(BHEAD gcdout1,startebuf) ) == 0 ) goto CalledFrom;
+		if ( ( gcdout = TakeExtraSymbols(BHEAD gcdout1,startebuf,extrasymbolbase) ) == 0 ) goto CalledFrom;
 		M_free(gcdout1,"gcdout");
 	}
 	else {
@@ -1246,14 +1246,14 @@ WORD *GCDfunction3(PHEAD WORD *in1, WORD *in2)
  		#[ PutExtraSymbols :
 */
 
-WORD *PutExtraSymbols(PHEAD WORD *in,WORD startebuf,int *actionflag)
+WORD *PutExtraSymbols(PHEAD WORD *in,WORD startebuf,int *actionflag,WORD *extrasymbolbase)
 {
 	WORD *termout = AT.WorkPointer;
 	int action;
 	*actionflag = 0;
 	NewSort(BHEAD0);
 	while ( *in ) {
-		if ( ( action = LocalConvertToPoly(BHEAD in,termout,startebuf,0) ) < 0 ) {
+		if ( ( action = LocalConvertToPoly(BHEAD in,termout,startebuf,0,extrasymbolbase) ) < 0 ) {
 			LowerSortLevel();
 			goto CalledFrom;
 		}
@@ -1275,7 +1275,7 @@ WORD *PutExtraSymbols(PHEAD WORD *in,WORD startebuf,int *actionflag)
  		#[ TakeExtraSymbols :
 */
 
-WORD *TakeExtraSymbols(PHEAD WORD *in,WORD startebuf)
+WORD *TakeExtraSymbols(PHEAD WORD *in,WORD startebuf,WORD extrasymbolbase)
 {
 	CBUF *C = cbuf+AC.cbufnum;
 	CBUF *CC = cbuf+AT.ebufnum;
@@ -1284,7 +1284,7 @@ WORD *TakeExtraSymbols(PHEAD WORD *in,WORD startebuf)
 	termout = AT.WorkPointer;
 	NewSort(BHEAD0);
 	while ( *in ) {
-		if ( ConvertFromPoly(BHEAD in,termout,numxsymbol,CC->numrhs-startebuf+numxsymbol,startebuf-numxsymbol,1) <= 0 ) {
+		if ( ConvertFromPoly(BHEAD in,termout,extrasymbolbase,CC->numrhs-startebuf+extrasymbolbase,startebuf-extrasymbolbase,1) <= 0 ) {
 			LowerSortLevel();
 			goto CalledFrom;
 		}
@@ -2851,7 +2851,7 @@ int DIVfunction(PHEAD WORD *term,WORD level,int par)
 	WORD *tstop, *tend, *r3, *rr, *rstop, tlength, rlength, newlength;
 	WORD *proper1, *proper2, *proper3 = 0, numdol = -1;
 	int numargs = 0, type1, type2, actionflag1, actionflag2;
-	WORD startebuf = cbuf[AT.ebufnum].numrhs;
+	WORD startebuf = cbuf[AT.ebufnum].numrhs, extrasymbolbase = -1;
 	int division = ( par <= 2 );  /* false for mul_ */
 	if ( par < 0 || par > 3 ) {
 /* INTERNAL_ERROR_EXCL_START */
@@ -2939,8 +2939,8 @@ divzero:;
 		M_free(arg1,"DIVfunction");
 		return(0);
 	}
-	if ( ( proper1 = PutExtraSymbols(BHEAD arg1,startebuf,&actionflag1) ) == 0 ) goto CalledFrom;
-	if ( ( proper2 = PutExtraSymbols(BHEAD arg2,startebuf,&actionflag2) ) == 0 ) goto CalledFrom;
+	if ( ( proper1 = PutExtraSymbols(BHEAD arg1,startebuf,&actionflag1,&extrasymbolbase) ) == 0 ) goto CalledFrom;
+	if ( ( proper2 = PutExtraSymbols(BHEAD arg2,startebuf,&actionflag2,&extrasymbolbase) ) == 0 ) goto CalledFrom;
 /*
 	if ( type2 == 0 ) M_free(arg2,"DIVfunction");
 	else {
@@ -2965,7 +2965,7 @@ divzero:;
 	else if ( par == 3 ) proper3 = poly_mul(BHEAD proper1, proper2);
 	if ( proper3 == 0 ) goto CalledFrom;
 	if ( actionflag1 || actionflag2 ) {
-		if ( ( arg3 = TakeExtraSymbols(BHEAD proper3,startebuf) ) == 0 ) goto CalledFrom;
+		if ( ( arg3 = TakeExtraSymbols(BHEAD proper3,startebuf,extrasymbolbase) ) == 0 ) goto CalledFrom;
 		M_free(proper3,"DIVfunction");
 	}
 	else {
diff --git a/sources/structs.h b/sources/structs.h
index 707e3e0f..6789af4d 100644
--- a/sources/structs.h
+++ b/sources/structs.h
@@ -1438,7 +1438,7 @@ struct M_const {
 #ifdef WITHPTHREADS
     pthread_rwlock_t handlelock;   /* (M) */
     pthread_mutex_t storefilelock; /* (M) */
-	pthread_mutex_t	sbuflock;      /* (M) Lock for writing in the AM.sbuffer */
+	pthread_mutex_t  sbuflock;      /* (M) Lock for the global AM.sbuffer */
     LONG    ThreadScratSize;       /* (M) Size of Fscr[0/2] buffers of the workers */
     LONG    ThreadScratOutSize;    /* (M) Size of Fscr[1] buffers of the workers */
 #endif
@@ -1843,7 +1843,7 @@ struct C_const {
     int     NoCompress;            /* (R) Controls native compression */
     int     IsFortran90;           /* Tells whether the Fortran is Fortran90 */
     int     MultiBracketLevels;    /* Number of elements in MultiBracketBuf */
-    int     topolynomialflag;      /* To avoid ToPolynomial and FactArg together */
+    int     topolynomialflag;      /* Module can add entries to the global extra-symbol buffer */
     int     ffbufnum;              /* Buffer number for user defined factorizations */
     int     OldFactArgFlag;
     int     MemDebugFlag;          /* Only used when MALLOCDEBUG in tools.c */