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