| #### D.15.6.21 ParToVar Procedure from library `finitediff.lib` (see finitediff_lib). Usage: ParToVar(f); f a poly in the basering; Return: type poly; gives back the poly obtained by substituting par(i) by var(i), for the first nvars(basering parameters. Does only make sense, if setinitials is executed before -> nvars <= npars. Is the opposite action to VarToPar, see example ParToVar; Example:
    LIB "finitediff.lib";
    list D="Ut","Ux","Uy","U";
    list V="t","x","y";
    list P="a","b";
    setinitials(V,D,P);////does not show the ring, as there is  no output
    basering;///does show the ring
    ==> // coefficients: QQ(I, T, Px, Py, Cx, Cy, Sx, Sy, a, b, dt, dx, dy) consi\
       dered as a field
    ==> // number of vars : 8
    ==> //        block   1 : ordering c
    ==> //        block   2 : ordering lp
    ==> //                  : names    i t x y cx cy sx sy
    ==> // quotient ring from ideal
    ==> _[1]=cy^2+sy^2-1
    ==> _[2]=cx^2+sx^2-1
    ==> _[3]=i^2+1
    poly f=t**3*cx**2-cy**2*dt+i**3*sx/dt*dx;
    f;
    ==> (dx)/(dt)*i^3*sx+t^3*cx^2+(-dt)*cy^2
    def g=VarToPar(f);
    g;
    ==> (I^3*Sx*dx+T^3*Cx^2*dt-Cy^2*dt^2)/(dt)
    def h=ParToVar(g);
    h==f;
    ==> 1

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