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equivalence(e1, e2)
Returns equivalence classes. In arrays e1 and
e2 (which are transformed to type long) the user
specifies which elements of a given set belong to the same equivalence
class: elements e1(i) and e2(i)
are in the same equivalence class for all i. The result of
this function is an array with the same dimensional structure as
e1 in which each element contains the lowest element number
in the class that the current element is in.
For example, if e1 = [2,4,6,3] and e2 = [3,5,7,6], then
equivalence(e1, e2) returns [2,4,2,2], indicating that
elements 2, 3, 6, and 7 belong to one equivalence class (with element 2
having the smallest element number) and elements 4 and 5 to another
(with element 4 the smallest).