### 19.7.2 3D parametric surface

To draw a parametric surface in ℝ3:

- plotparam takes two mandatory arguments and two optional arguments:
  - exprs, a list of three expressions involving two parameters.
  - vars, a list of the parameters, optionally with a range var=a..b.
  - Optionally, ustep=n, to set the discretization step of the first parameter.
  - Optionally, vstep=m, to set the discretization step of the second parameter.
- plotparam(exprs,vars⟨,ustep=n,vstep=m⟩) draws the parametric representation of the surface.

#### Examples

plotparam([v*cos(u),v*sin(u),v],[u,v])

plotparam([v*cos(u),v*sin(u),v],[u=0..pi,v=0..3])

plotparam([v*cos(u),v*sin(u),v],[u=0..pi,v=0..3],ustep=0.5,vstep=0.5)