1.2.12 `sm1.mul'
----------------

sm1.mul(F,G,V|proc=P)
     ::  ask the sm1 server to multiply F and G in the ring of
     differential operators over V.

RETURN
     Polynomial or List

P
     Number

F, G
     Polynomial or List

V
     List

   * Ask the sm1 server to multiply F and G in the ring of differential
     operators over V.

   * `sm1.mul_h' is for homogenized Weyl algebra.

   * BUG: `sm1.mul(p0*dp0,1,[p0])' returns `dp0*p0+1'.  A variable
     order such that d-variables come after non-d-variables is
     necessary for the correct computation.

     [277] sm1.mul(dx,x,[x]);
     x*dx+1
     [278] sm1.mul([x,y],[1,2],[x,y]);
     x+2*y
     [279] sm1.mul([[1,2],[3,4]],[[x,y],[1,2]],[x,y]);
     [[x+2,y+4],[3*x+4,3*y+8]]

