A sub-domain is a piece of the domain which is simply connected and delimited by boundaries.
Each sub-domain has a region number assigned to it. This is done
by Gfem, not by the user. Every time border
is called, an internal
number ngt
is incremented by 1. Then when the key word border
is
invoked the last edge of this portion of boundary assigns this number
to the triangle which lies on its left. Finally all triangles which are in
this subdomain are reassigned this number.
At the end of the triangulation process, each triangle has a well defined
number ngt
. The number region is a piecewise linear continuous interpolation
at the vertices of ngt
. To be exact, the value of region at a
vertex is the value of ngt at
,
except if precise is set in which case region is equal to
ngt.