Inconsistency of built-in containers functional interface

[Night-walker]

Some code section methods yield item: @T, other -- item: invar<@T> with no clue as to why each variant was chosen. It definitely looks like invar violation when, for instance, list.collect(){} accepting invar list yields non-invar items.

[Night-walker]

(Also, invar item: @T is preferable to item: invar<@T>)