Isn't merge commutative?

[Kyung-Kook Park]

I thought mercurial merge was commutative.
But today I met a non-commutative merging case from my repository.
The result of merging branch A from branch B was
different from that of merging branch B from branch A.

What actually happened is modification event in branch A
was overwritten by add event in branch B,
which resulted in roll-back of several files.
But the files are merged as intended, when I changed the
direction of merge.

How can this happen?

Kyung