Function: setunion
Section: linear_algebra
C-Name: setunion
Prototype: GG
Help: setunion(x,y): union of the sets x and y.
Description:
 (vec, vec):vec        setunion($1, $2)
Doc: union of the two sets $x$ and $y$ (see \kbd{setisset}).
 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.
