:import std/Number .
:import std/List .
y* [[[0 1] <$> 0] ([[1 <! ([[1 2 0]] <$> 0)]] <$> 0)]
# --- example usage ---
# mutual recurrence relation of odd?/even?
# even x = if x == 0 then true else odd? (x-1)
g [[[=?0 [[1]] (1 --0)]]]
# odd x = if x == 0 then false else even? (x-1)
h [[[=?0 [[0]] (2 --0)]]]
even? ^(y* (g : {}h))
odd? _(y* (g : {}h))
:test (even? (+5)) ([[0]])
:test (odd? (+5)) ([[1]])
main [[0]]