\DOC set_diff

\TYPE {set_diff : ''a list -> ''a list -> ''a list}

\SYNOPSIS
Computes the set-theoretic difference of two `sets'.

\KEYWORDS
list, set, eqtype.

\DESCRIBE
{set_diff l1 l2} returns a list consisting of those elements of {l1} that do
not appear in {l2}. It is identical to {Lib.subtract}.

\FAILURE
Never fails.

\EXAMPLE
{
- set_diff [] [1,2];
> val it = [] : int list

- set_diff [1,2,3] [2,1];
> val it = [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}.

ML equality types are used in the implementation of {union} and its kin.
This limits its applicability to types that allow equality. For other
types, typically abstract ones, use the `op_' variants.

\SEEALSO
Lib.op_set_diff, Lib.subtract, Lib.mk_set, Lib.set_eq, Lib.union, Lib.intersect.
\ENDDOC