let! a = 34

assert 34 == a
assert 102 == 3 * a

a = 9

a = a + 1

assert 11 == a + 1

assert_static_fail a and true
assert_static_fail a = false

let b = 7

assert 7 == b
assert 42 == 6 * b
assert 17 == a + b

assert_static_fail b = 4