| ##### 7.5.15.0. invertNcfrac Procedure from library `ncfrac.lib` (see ncfrac_lib). Usage: invertNcfrac(frac), ncfrac frac Purpose: compute the inverse of frac Return: ncfrac Note: returns the zero fraction if frac is not invertible Example:
    LIB "ncfrac.lib";
    ==> // ** redefining testNcfrac (LIB "ncfrac.lib";) ./examples/invertNcfrac.s\
       ing: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 frac1 = list([y,y+1,0,0], loc);
    // frac1 is invertible
    ncfrac inv = invertNcfrac(frac1);
    inv;
    ==> left repr.: (y+1,y)
    ==> right repr.: (0,0)
    ==>
    ncfrac frac2 = list([y,x+1,0,0], loc);
    // frac2 is not invertible
    inv = invertNcfrac(frac2);
    inv;
    ==> left repr.: (1,0)
    ==> right repr.: (0,1)
    ==>

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