| #### D.15.5.5 laxfrX Procedure from library `finitediff.lib` (see finitediff_lib). Usage: laxfrX(U1,U2,var); U1, U2 are the names of occuring derivatives, var is a variable in the basering; Return: type vector; gives a predefined approximation of the Lax-Friedrich-approximation for the derivation in one of the spatial variables as often used in literature; Note: see also laxfrT, setinitials, scheme; Warning: laxfrX is not to be interchanged with laxfrT Example:
    LIB "finitediff.lib";
    list D="Ux","Ut","U";
    list P="a";
    list V="t","x";
    setinitials(V,D,P);
    laxfrX(Ux,U,x);
    ==> [(2*dx)*x,0,-x^2+1]

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