combine : 'a list * 'b list -> ('a * 'b) list
Transforms a pair of lists into a list of pairs.
combine ([x1,...,xn],[y1,...,yn]) returns [(x1,y1),...,(xn,yn)].
Fails if the two lists are of different lengths.
Has much the same effect as the SML Basis function ListPair.zip
except that it fails if the arguments are not of equal length. Also
note that zip is a curried version of combine