>>> from enthought.traits.api import HasTraits, Trait
>>> from enthought.traits.api import TraitPrefixList
>>> class Alien(HasTraits):
...   heads = Trait('one', \
...                 TraitPrefixList(['one', 'two', 'three']))
... 
>>> alf = Alien()
>>> alf.heads = 'o'  
>>> print alf.heads
one
>>> alf.heads = 'tw' 
>>> print alf.heads
two 
>>> alf.heads = 't'  # Error, not a unique prefix
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "c:\wrk\src\lib\enthought\traits\trait_handlers.py", line 
601, in validate
    self.error( object, name, self.repr( value ) )
  File "c:\wrk\src\lib\enthought\traits\trait_handlers.py", line 
90, in error
    raise TraitError, ( object, name, self.info(), value )
enthought.traits.trait_errors.TraitError: The 'heads' trait of an 
Alien instance must be 'one' or 'two' or 'three' (or any unique 
prefix), but a value of 't' was specified.
