The window "Domain" displays a domain and a point
v0) in the domain.
The window "Surface: X(u, v)" shows the surface X(u, v) and the point X(u0, v0).
The window "Coefficients of 2nd Fundamental Form" graphs L11(u,v),
L12(u,v) = L21(u,v), and L22(u,v) in
red, green, and cyan, respectively.
It is possible to click and drag
point (u0, v0) in the
domain, and it is possible to change X(u, v) in the control window.