Creating a Surface Between Two Curves.
The curve through points A, B, C, and D is defined by a parametric function
a(t).
The curve through points E, F, G, and H is defined by a parametric function b(t).
The idea is to "stretch" the surface from one curve to the other, which is the purpose of the u parameter.
The u parameter varies from 0 to 1 (or in this app, 0 to c which varies from 0 to 1). When u = 1, the surface is all a(t). When u = 0, the surface is all b(t). When u = 0.4, the surface stretches 40% of the way from b(t) to a(t).
The parameter t varies from 0 to 1 for splines. This can vary as freely as you like.
This is the command:
Surface(u x(a(t)) + (1 - u) x(b(t)), u y(a(t)) + (1 - u) y(b(t)), u z(a(t)) + (1 - u) z(b(t)), t, 0, e, u, 0, c)