The objects are assumed to have the same class, and the result will have the class of one of them. The combined object will contain f(p,q) => g(b,c) when x : p => b and y : q => c, and the function h is used to combine values when key collisions occur in the result, as with merge. The function h should be a function of two arguments; it may assume that its first argument will be the value accumulated so far, and its second argument will be the result g(b,c) from a single pair of values. Normally h will be an associative and commutative function.
The result is mutable if and only if x or y is.
This function can be used for multiplying polynomials, where it will look something like this: combine(x,y,monomialTimes,coeffTimes,coeffPlus).
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