| #### 7.10.3.7 lpSubstitute Procedure from library `fpaprops.lib` (see fpaprops_lib). Usage: lpSubstitute(f,s1,s2[,G]); f poly, s1 list (ideal) of variables to replace, s2 list (ideal) of polynomials to replace with, G optional ideal to reduce with. Return: poly, the substituted polynomial Assume: - basering is a Letterplace ring - s1 contains a subset of the set of variables - s2 and s1 are of the same size - G is a Groebner basis, - the current ring has a sufficient degbound (which also can be calculated with lpCalcSubstDegBound()) Note: the procedure implements the image of a polynomial f under an endomorphism of a free algebra, defined by s1 and s2: variables, not present in s1, are left unchanged; variable s1[k] is mapped to a polynomial s2[k]. - An optional ideal G extends the endomorphism as above to the morphism into the factor algebra K<X>/G. Example:
    LIB "fpaprops.lib";
    ring r = 0,(x,y,z),dp;
    def R = freeAlgebra(r, 4);
    setring R;
    ideal G = x*y; // optional
    poly f = 3*x*x+y*x;
    ideal s1 = x, y;
    ideal s2 = y*z*z, x; // i.e. x --> yzz and y --> x
    // the substitution probably needs a higher degbound
    int minDegBound = lpCalcSubstDegBound(f,s1,s2);
    minDegBound; // thus the bound needs to be increased
    ==> 9
    setring r; // back to original r
    def R1 = freeAlgebra(r, minDegBound);
    setring R1;
    lpSubstitute(imap(R,f), imap(R,s1), imap(R,s2));
    ==> 3*y*z*z*y*z*z+x*y*z*z
    // the last parameter is optional; above it was G=<xy>
    // the output will be reduced with respect to G
    lpSubstitute(imap(R,f), imap(R,s1), imap(R,s2), imap(R,G));
    ==> 3*y*z*z*y*z*z

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