, and normal vector ~~ at parameters U,V.
DerivAndNorm(myclass; S: TheSurface; U,V: Real from Standard;
P : out Pnt from gp;
d1u,d1v: out Vec from gp;
N : out Vec from gp);
---Purpose: Computes the point ~~

~~, and normal vector ~~~~ at parameters U,V.
NormAndDn(myclass; S: TheSurface; U,V: Real from Standard;
P : out Pnt from gp;
N,Dnu,Dnv: out Vec from gp);
---Purpose: Computes the point ~~

~~, normal vector ~~~~ at parameters U,V.
end SurfProps;
~~