address the use case of overlapping positive positions

[protobufel]

Consider (a | b)(0) c(-1) (d | f | e)(2) a(-1) and treat duplicate positive positions as the case of OR-ed values, so that {a, b, c, d, f, e, a} with the corresponding {0, 0, -1, 2, 2, 2, -1} would not cause IllegalArgumentException as of now.