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\DOC subtract'
\TYPE {subtract' : ('a -> 'a -> bool) -> 'a list -> 'a list -> 'a list}
\SYNOPSIS
Subtraction of sets modulo an equivalence.
\DESCRIBE
The call {subtract' r l1 l2} removes from the list {l1} all elements {x} such
that there is an {x'} in {l2} with {r x x'}. 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 {subtract} is the special case
where the relation is just equality.
\FAILURE
Fails only if the function {r} fails.
\EXAMPLE
{
# subtract' (fun x y -> abs(x) = abs(y)) [-1; 2; 1] [-2; -3; 4; -4];;
val it : int list = [-1; 1]
}
\USES
Maintaining sets modulo an equivalence such as alpha-equivalence.
\SEEALSO
insert', mem', union, union', unions'.
\ENDDOC
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