get(xs, obj, @optional default = false)
Return tail of the first element of the list xs whose head is equivalent
to obj. If no such element is found, return default.
get([1:2, 3:4], 1)
// 2
get([1:2, 3:4], 5)
// false
get([1:2, 3:4], 5, 6)
// 6