(l: List) each: (b: Block) :=
  { l map: b in-context
    l
  } call

[] includes?: List := False
(x: List) includes?: (y: List) :=
  if: (x (take: y length) == y)
    then: { True }
    else: { x tail includes?: y }

[] join: List := []
[x] join: List := x
(x . xs) join: (d: List) :=
  x .. d .. (xs join: d)