@main = a
  & @sum ~ (20 (0 a))

@sum = (?(((a a) @sum__C0) b) b)

@sum__C0 = ({c a} ({$([*2] $([+1] d)) $([*2] $([+0] b))} f))
  &! @sum ~ (a (b $([+] $(e f))))
  &! @sum ~ (c (d e))