Checks that required attributes have been defined,
           attributes w/default cannot be required.   Checks
           all defined attributes are legal, attribute 
           references are not subject to this test.

def __checkAttributes(self):
        """Checks that required attributes have been defined,
           attributes w/default cannot be required.   Checks
           all defined attributes are legal, attribute 
           references are not subject to this test.
        """
        for a in self.__class__.required:
            if not self.attributes.has_key(a):
                raise SchemaError,\
                    'class instance %s, missing required attribute %s'\
                    %(self.__class__, a)
        for a,v in self.attributes.items():
            # attribute #other, ie. not in empty namespace
            if type(v) is dict:
                continue
            
            # predefined prefixes xmlns, xml
            if a in (XMLSchemaComponent.xmlns, XMLNS.XML):
                continue
            
            if (a not in self.__class__.attributes.keys()) and not\
                (self.isAttribute() and self.isReference()):
                raise SchemaError, '%s, unknown attribute(%s,%s)' \
                    %(self.getItemTrace(), a, self.attributes[a])