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D.15.25.11 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";)
==> // ** redefining testNcloc (    LIB "ncloc.lib";)
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)
==>