@gen = (?(((a a) @gen__C0) b) b)

@gen__C0 = ({a d} ({$([*2] $([+1] b)) $([*2] e)} (c f)))
  &! @gen ~ (a (b c))
  &! @gen ~ (d (e f))

@main = a
  & @sum ~ (20 (@main__C0 a))

@main__C0 = a
  & @gen ~ (20 (0 a))

@sum = (?(((* 1) @sum__C0) a) a)

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