| ##### 7.5.15.0. isInvertibleNcfrac Procedure from library `ncfrac.lib` (see ncfrac_lib). Usage: isInvertibleNcfrac(frac), ncfrac frac Purpose: checks if frac is invertible Return: int, 1 if frac is invertible, 0 otherwise Example:
    LIB "ncfrac.lib";
    ==> // ** redefining testNcfrac (LIB "ncfrac.lib";) ./examples/isInvertibleNc\
       frac.sing:1
    ==> // ** redefining testNcloc (    LIB "ncloc.lib";) ncfrac.lib::mod_init:11\
       3
    ring R = 0,(x,y,Dx,Dy),dp;
    def S = Weyl();
    setring S;
    ncloc loc = intvec(2);
    ncfrac frac = list([y,y+1,0,0], loc);
    isInvertibleNcfrac(frac);
    ==> 1
    frac = list([y,x+1,0,0], loc);
    isInvertibleNcfrac(frac);
    ==> 0

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