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 $y$.
 The function also works if both $x$ and $y$ are vectors of strictly increasing
 entries, according to \kbd{<}); in that case we return a vector of strictly
 increasing entries, not a set. Otherwise, the result is undefined.
