## Description

mergePairs(x,y,f) merges sorted lists of pairs.

It merges

x and

y, which should be lists of pairs

(k,v) arranged in increasing order according to the key

k. The result will be a list of pairs, also arranged in increasing order, each of which is either from

x or from

y, or in the case where a key

k occurs in both, with say

(k,v) in

x and

(k,w) in

y, then the result will contain the pair

(k,f(v,w)). Thus the function

f is used for combining the values when the keys collide. The class of the result is taken to be the minimal common ancestor of the class of

x and the class of

y.