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\DOC union'
\TYPE {union' : ('a -> 'a -> bool) -> 'a list -> 'a list -> 'a list}
\SYNOPSIS
Union of sets modulo an equivalence.
\DESCRIBE
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
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.
\SEEALSO
insert', mem', subtract', union, unions'.
\ENDDOC
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