Function: setminus
Section: linear_algebra
C-Name: setminus
Prototype: GG
Help: setminus(x,y): set of elements of x not belonging to y.
Description:
 (vec, vec):vec        setminus($1, $2)
Doc: difference of the two sets $x$ and $y$ (see \kbd{setisset}),
 i.e.~set of elements of $x$ which do not belong to $y$.
 If $x$ or $y$ is not a set, the result is undefined.
