Topographer,
Concerning the oasis, holes or island however we name them,
it can be done with the program as it is.
The holes is actually a polyline that encircles an area without
points.
This is actually a close breakline without any points inside.
Only thing missing is as you create the CDT, 3dfaces will be created
inside the breaklines. In other word the inside of your polyline
defining the breakline will be triangulated also.
All you need to do is to erase these inside triangles
manually upon completion. Should be easy if you
use the breakline as a selecting polygon.
ymg