list: cannot .set(#, undefined)

[issa-tseng]

because List#set will take an undefined value parameter to mean that you want a partially applied function back, it's not possible to set the value to undefined. you can only set to null.

this is sort of unfortunate, but what makes it really bad is that ergo mapping a list value to undefined would just result in a silent failure. we've now patched it so the mapper will fall back on null if undefined is given, but this is definitely not great.

[issa-tseng]

use arguments.length ?