class UnknownPrefix(Exception):
    '''Raised if a given prefix does not map to any items'''
    def __init__(self,prefix,*args):
        Exception.__init__(self,*args)
        self.prefix = prefix

class AmbiguousPrefix(Exception):
    '''Raised if a given prefix maps to multiple items'''
    def __init__(self,prefix,choices,*args):
        Exception.__init__(self,*args)
        self.prefix = prefix
        self.choices = choices