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.