Module solid 

Solids from closed shells, outer boundaries told from voids by which shell lies inside which.

A file or a sewing hands over closed shells with nothing saying which bounds a part and which a cavity in one. Nesting says it: a shell inside no other is a part’s outer boundary, a shell inside one is a void of it, a shell inside two is the outer boundary of a part inside that cavity, and so on down.

Functions

solids_from_shells

The solids shells bound: one where a single shell is outermost, else a compound of them, one per outermost shell and per shell lying in a void.