1.2.48 `sm1.gb_reduction'
-------------------------

sm1.gb_reduction(F,A)
     :  It returns a reduced form of F in terms of A by using  a normal
     form algorithm. h[1,1](D)-homogenization is used.
Example:

      input
        F=[2*(h-x-y)*dx+h^2,2*(h-x-y)*dy+h^2]$
        FF=[F,"x,y",[[dx,1,dy,1],[x,-1,y,-1,dx,1,dy,1]]]$
        sm1.gb_reduction((h-x-y)^2*dx*dy,FF);

