1.2.8 `sm1.wTensor0'
--------------------

sm1.wTensor0([F,G,V,W]|proc=P)
     ::   It computes the D-module theoretic 0-th tensor product     of
     F and G.

RETURN
     List

P
     Number

F, G, V, W
     List

   *    It returns the D-module theoretic 0-th tensor product    of F
     and G.

   *   V is a list of variables.    W is a list of weights.  The
     integer W[i] is   the weight of the variable V[i].

   *    `sm1.wTensor0' calls `wRestriction0' of `ox_sm1',    which
     requires a generic weight     vector W to compute the restriction.
        If W is not generic, the computation fails.

   * Let F and G be solutions of F and G respectively.  Intuitively
     speaking, the 0-th tensor product is a system of differential
     equations which annihilates the function FG.

   * The answer is a submodule of a free module D^r in general even if
     the inputs F and G are left ideals of D.

     [258]  sm1.wTensor0([[x*dx -1, y*dy -4],[dx+dy,dx-dy^2],[x,y],[1,2]]);
     [[-y*x*dx-y*x*dy+4*x+y],[5*x*dx^2+5*x*dx+2*y*dy^2+(-2*y-6)*dy+3],
      [-25*x*dx+(-5*y*x-2*y^2)*dy^2+((5*y+15)*x+2*y^2+16*y)*dy-20*x-8*y-15],
      [y^2*dy^2+(-y^2-8*y)*dy+4*y+20]]

