def select(self, fn, tag=None):
return self.subtess(self.map(fn), tag=tag)<EOL>
tess.select(fn) is equivalent to tess.subtess(tess.properties.map(fn)); any vertex whose
  property data yields True or 1 will be included in the new subtess and all other vertices
  will be excluded.
The optional parameter tag is used identically as in tess.subtess().