quadruple_of_list
Lib.quadruple_of_list : 'a list -> 'a * 'a * 'a * 'a
Turns a four-element list into a quadruple.
quadruple_of_list [x1, x2, x3, x4] returns (x1, x2, x3, x4).
Failure
Fails if applied to a list that is not of length 4.