\DOC op_set_diff

\TYPE {op_set_diff : ('a -> 'a -> bool) -> 'a list -> 'a list -> 'a list}

\SYNOPSIS
Computes the set-theoretic difference of two `sets', modulo a
supplied relation.

\KEYWORDS
list, set.

\DESCRIBE
{op_set_diff eq l1 l2} returns a list consisting of those elements
of {l1} that are not {eq} to some element of {l2}.

\FAILURE
Fails if an application of {eq} fails.

\EXAMPLE
{
- op_set_diff (fn x => fn y => x mod 2 = y mod 2) [1,2,3] [4,5,6];
> val it = [] : int list

- op_set_diff (fn x => fn y => x mod 2 = y mod 2) [1,2,3] [2,4,6,8];
> val it = [1, 3] : int list
}


\COMMENTS
The order in which the elements occur in the resulting list should
not be depended upon.

A high-performance implementation of finite sets may be found in
structure {HOLset}.

\SEEALSO
Lib.set_diff, Lib.op_mem, Lib.op_insert, Lib.op_union, Lib.op_U, Lib.op_mk_set, Lib.op_intersect.
\ENDDOC