/* ---------------------------------- */
/* save as x_idrst.h                  */
/* ---------------------------------- */
double implicitDu_r(
double (*P_u)(double x, double y, double z),
double (*P_x)(double r, double s, double t),
double (*P_y)(double r, double s, double t),
double (*P_z)(double r, double s, double t),
pt3d  Prst,
double   h
)
{
pt3d Pxyz =          {(*P_x)(Prst.x,Prst.y,Prst.z),
                      (*P_y)(Prst.x,Prst.y,Prst.z),
                      (*P_z)(Prst.x,Prst.y,Prst.z)};
                              
 return(fxyz_x((*P_u),Pxyz,h)*fxyz_x((*P_x),Prst,h)+
        fxyz_y((*P_u),Pxyz,h)*fxyz_x((*P_y),Prst,h)+
        fxyz_z((*P_u),Pxyz,h)*fxyz_x((*P_z),Prst,h)
       );
}
/* ---------------------------------- */
double implicitDu_s(
double (*P_u)(double x, double y, double z),
double (*P_x)(double r, double s, double t),
double (*P_y)(double r, double s, double t),
double (*P_z)(double r, double s, double t),
pt3d  Prst,
double   h
)
{
pt3d  Pxyz =         {(*P_x)(Prst.x,Prst.y,Prst.z),
                      (*P_y)(Prst.x,Prst.y,Prst.z),
                      (*P_z)(Prst.x,Prst.y,Prst.z)};
                              
 return(fxyz_x((*P_u),Pxyz,h)*fxyz_y((*P_x),Prst,h)+
        fxyz_y((*P_u),Pxyz,h)*fxyz_y((*P_y),Prst,h)+
        fxyz_z((*P_u),Pxyz,h)*fxyz_y((*P_z),Prst,h)
       );
}
/* ---------------------------------- */
double implicitDu_t(
double (*P_u)(double x, double y, double z),
double (*P_x)(double r, double s, double t),
double (*P_y)(double r, double s, double t),
double (*P_z)(double r, double s, double t),
pt3d  Prst,
double   h
)
{
pt3d  Pxyz =         {(*P_x)(Prst.x,Prst.y,Prst.z),
                      (*P_y)(Prst.x,Prst.y,Prst.z),
                      (*P_z)(Prst.x,Prst.y,Prst.z)};
                              
 return(fxyz_x((*P_u),Pxyz,h)*fxyz_z((*P_x),Prst,h)+
        fxyz_y((*P_u),Pxyz,h)*fxyz_z((*P_y),Prst,h)+
        fxyz_z((*P_u),Pxyz,h)*fxyz_z((*P_z),Prst,h)
       );
}
/* ---------------------------------- */
/* ---------------------------------- */