GFem language documentation -- 3.3.3 Geometric variables, inner and region bdy, normal

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### 3.3.3 Geometric variables, inner and region bdy, normal

- `x,y` refers to the coordinates in the domain
- `ib` refers to the boundary identification number; it is zero inside the domain.
- `nx` and `ny` refer to the x-y components of the normal on the boundary vertices; it is zero on all inner vertices.
- `region` refers to the domain identification number which is itself based on an internal number, ngt, assigned to each triangle by the triangulation constructor.

Inner boundaries which divide the domain into several simply connected components are useful to define piecewise discontinuous functions such as dielectric constants, Young modulus...

Inner boundaries may meet other boundaries only at their vertices. Such inner boundaries will split the domain in several sub-domains.