| #### D.15.4.22 qepcad Procedure from library `finitediff.lib` (see finitediff_lib). Usage: qepcad(f); f a poly in the basering; Return: type list; gives back some constraints that are equivalent to f<1 (computed by QEPCAD); 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),laxfrX(Ux,U,x),laxfrT(Ut,U,x));
    s1;
    ==> (-a*dt+dx)/(2*dx)*x^2+(a*dt+dx)/(2*dx)
    def s2=errormap(s1);
    s2;
    ==> (-a*dt+dx)/(dx)*i*cx*sx+(a*dt-dx)/(dx)*sx^2+1
    def s3=ComplexValue(s2);s3;
    ==> (a^2*dt^2-dx^2)/(dx^2)*sx^2+1
    qepcad(s3);
    ==> 0
    ==> 0
    ==> 0
    ==> sh: qepcad: Kommando nicht gefunden.
    ==> sh: qepcadfilter.pl: Kommando nicht gefunden.
    ==> 32512
    ==>
    ==> Try manually

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