Imposes a user-defined constraint on triangle size. There are two ways to use this feature. One is to edit the triunsuitable() procedure in triangle.c to encode any constraint you like, then recompile Triangle. The other is to compile triangle.c with the EXTERNAL_TEST symbol set (compiler switch -DEXTERNAL_TEST), then link Triangle with a separate object file that implements triunsuitable(). In either case, the -u switch causes the user- defined test to be applied to every triangle.