error: cannot type Assig("fact",Int("1"))
    inferred types of subterms: 
    typed "fact" as String, Int, Id, <string>
    typed Int("1") as Exp
error: cannot type Mul("x",Var("fact"))
    inferred types of subterms: 
    typed "x" as String, Int, Id, <string>
    typed Var("fact") as Exp