union' : ('a -> 'a -> bool) -> 'a list -> 'a list -> 'a list
SYNOPSIS
Union of sets modulo an equivalence.
DESCRIPTION
The call union' r l1 l2 appends to the list l2 all those elements x of
l1 for which there is not already an equivalent x' with r x x' in l2 or
earlier in l1. If l1 and l2 were free of equivalents under r, the
resulting list will be too, so this is a set operation modulo an equivalence.
The function union is the special case where the relation is just equality.
FAILURE CONDITIONS
Fails only if the function r fails.
EXAMPLE
# union' (fun x y -> abs(x) = abs(y)) [-1; 2; 1] [-2; -3; 4; -4];;
val it : int list = [1; -2; -3; 4; -4]
USES
Maintaining sets modulo an equivalence such as alpha-equivalence.