/* ---------------------------------- */
/* save as x_nxy.h                     */
/* ---------------------------------- */
/*           (-f_xi-f_yj+k)           
        n =  ------------            
           [(f_x)^2+(f_y)^2+1]^1/2 

*/
/* ---------------------------------- */
v3d n_dxdy(
double (*P_f)(double x, double y),   /* g(x,y,z) = z - f(x,y) */
double  h,
p2d p
)
{
v3d  n;
double    d;

          d = sqrt( pow(fxy_x((*P_f),h,p),2) +
                    pow(fxy_y((*P_f),h,p),2) +
                     1);

          n.i =  - fxy_x((*P_f),h,p) / d;
          n.j =  - fxy_y((*P_f),h,p) / d;
          n.k =                    1 / d;

     return(n);
}
/* ---------------------------------- */
/* ---------------------------------- */