| #### D.15.8.3 scheme Procedure from library `finitediff.lib` (see finitediff_lib). Usage: scheme([v1,..,vn]); v1,..,vn of type vector Return: poly Purpose: performs substitutions by the means of Groebner basis computation of the submodule, generated by the input vectors, then intersects the intermediate result with the suitable component in order to get a finite difference scheme Note: works only for a single PDE, for the case of a system use `matrixsystem` Example:
    LIB "finitediff.lib";
    list D="Ux","Ut","U";
    list P="a";
    list V="t","x";
    setinitials(V,D,P);
    def s1=scheme(u(Ut)+a*u(Ux),backward(Ux,U,x),forward(Ut,U,t));
    s1;
    ==> (-a*dt+dx)/(dx)*x+(a*dt)/(dx)

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