Expr.expected

test1: Just (Const 2)
test2: Nothing
test3: Nothing
test4: Just (Const 2 :*: Const 4)
test5: Just (Const 4 :*: Const 2)
test6: Nothing
test7: Just (Const 4 :*: Const 2 :+: Const 7)
test8: Just (Const 2 :+: Const 1)
test9: Just (Const 2 :*: Const 3 :+: Const 2 :*: Const 4)
test10: Just (Const 1 :*: Const 2 :+: Const 1 :*: Const 3)
test11: Just (Const 2)