crazy recursive define of is_even and is_odd in nim

recur.nim

# predicate zero or not
proc zero(n : int) : bool =
    return n==0

# simple subtract 1
proc sub1(n : int) : int =
    return n-1

# forward defs since this is recursive
proc is_odd(n : int) : bool;
proc is_even(n : int) : bool;

proc is_even(n : int) : bool =
    # echo("is even called",n)
    return zero(n) or is_odd(sub1(n))

proc is_odd(n : int) : bool =
    # echo("is odd called",n)
    return not zero(n) and is_even(sub1(n))


echo( is_even(10))