def to_a
        a = []
        keys.each { |k| a.push [ k, self.fetch( k ) ] }
        a
    end
def to_a
        a = []
        keys.each { |k| a.push [ k, self.fetch( k ) ] }
        a
    end
:call-seq:
   ydbm.to_a -> array

 Converts the contents of the database to an array of [key, value] arrays,
 and returns it.