#$ Not mapping when instantiating function (during interpretation) causes `a` to not be mapped and fail when entering `just()`.
# 1

fn just(x): Just(x)

fn entries (self Slice a) -> Iter.MapMaybe (Slice.SliceIter a) a a:
	self into-iter() Iter.map-maybe(just)

xxs = Array.as-slice(&[1])
xs = entries(xxs as Slice I32)
xs for-each(println)