my case is even more simple (position is 2)

2/ I am searching for a fast / efficient way to use it at low level

1/ I don't see how to do it simply
2/ I am searching for a fast / efficient way to use it at low level

inserting-an-element-in-a-list-at-position-n

here a good one ( slice join join ) but too complex

still wrong you are doing a replace
here a good one ( slice join join ) but too complex


I don't know what the most efficient one (depend on zl implementation)

from your I simplified mine
I don't know what the most efficient one (depend on zl implementation)
but I'd like better (in term of CPU)

next time I'll read the first word of the post twice.

For the most efficient one, I don't know, repeat 2000 times the operation (uzi) and compare cpu time (see cpuclock helpfile)

inserting an element in a list at position n