def invert
        h = {}
        keys.each { |k| h[ self.fetch( k ) ] = k }
        h
    end
def invert
        h = {}
        keys.each { |k| h[ self.fetch( k ) ] = k }
        h
    end
:call-seq:
   ydbm.invert -> hash

 Returns a Hash (not a DBM database) created by using each value in the
 database as a key, with the corresponding key as its value.

 Note that all values in the hash will be Strings, but the keys will be
 actual objects.