MAYBE

Problem:
 and(tt(),tt()) -> tt()
 is_nat(0()) -> tt()
 is_nat(s(x)) -> is_nat(x)
 is_natlist(nil()) -> tt()
 is_natlist(cons(x,xs)) -> and(is_nat(x),is_natlist(xs))
 from(x) -> fromCond(is_natlist(x),x)
 fromCond(tt(),cons(x,xs)) -> from(cons(s(x),cons(x,xs)))

Proof:
 Open