Create a pipe with varying radii at specified loci.

Input parameters:

1. Rl → Rail curve at the centre of pipe.
2. Lc → Loci of specified radii.
3. Rd → Radii at loci.
4. Cp → Cap pipe ends with flat surfaces.
   •  True → Cap
   • False → Open

Output parameters:

1. Pi → Variable pipe surface around rail curve.