I don't know, why i can take id from primnum before mirror node and don't take primnum from id after mirror?
I want my curve doesn't change primnum after mirror.
one curve = one primitive

i uses @id = @primnum and its working
but @primnum = @id doesn't work.
i need it for a planarpath from curve don't make extra groups.

@primnum as well as @pointnum etc is a pseudo-attribute that can not have double numbers, if you have 5 prims, primnum will always count up from 0 to 4, you can only influence the order.

@id is a "real" attribute so if you add it before the mirror, it will get mirrored as you would expect and you can have multiple prims with the same values. @primnum however would just change to take the number of primitives and count up from 0 for each prim.

you could try the sort sop and sort by attribute (or any other method) if that gives you your desired behaviour