Singular

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