memq(x, xs)
Return the first tail of the list xs whose head is equivalent to x.
Return false if x is not found in xs.
The equivalence test for memq is is_eq.
memq(2, [1, 2, 3, 4, 5])
// [2, 3, 4, 5]
memq("hi", [1, 2, "hi", 4, 5])
// false