General question about concat

[fhalde]

Hi, I came across this implementation of yours from here

Amazing article! For my understanding, I decided to write it in Clojure but fumbled upon one aspect of concat

A multiset can have repetitive keys

[('cat', 4), ('dog', 2), ('elephant', -4), ('elephant', 4)]

When implementing concat, how do I decide which keys to merge it against? For e.g.

[('cat', 4), ('dog', 2), ('elephant', -4), ('elephant', 4)].concat([('elephant', 1)])

has 3 possibilities

#1 [('cat', 4), ('dog', 2), ('elephant', -3), ('elephant', 4)]
#2 [('cat', 4), ('dog', 2), ('elephant', -4), ('elephant', 5)]
#3 [('cat', 4), ('dog', 2), ('elephant', -4), ('elephant', 4), ('elephant', 1)]

Now, I know it doesn't matter since consolidating them results in the same outcome, so what would be my basis to make a choice?