Return true if obj is a vector, else return false.
Return true if vec is empty, i.e. its length is 0, else return false.
Return true if the vectors vec … have equal lengths and
equal elements according to elt=?. elt=? is always applied
to two arguments. Element comparison must be consistent with eq?
in the following sense: if (eq? a b) returns true, then
(elt=? a b) must also return true. The order in which
comparisons are performed is unspecified.