The python documentation on "Built-in Exceptions": <http://docs.python.org/library/exceptions.html> reads: ``In user defined base classes, abstract methods should raise this exception when they require derived classes to override the method.'' But existing NLTK code didn't consistently follow that best practice, sometimes using 'AssertionError' or 'assert' in similar situations. Fix that. Also, make error messages less verbose, for consistency with other existing code.
The CVS-style $Id$ lines are pretty much useless with git; and anyway, using them would require adding proper "ident" directives to the .gitattributes file -- and we don't have such directives. This change removes some occurrences of $Id$ lines that have been missed by yesterday's commit 44745e2.
…gative values to zero
… Exceptions were introduced. Documentation add to all four classes. Examples and doctests added to explain the interface