def add_property(name, value = nil, &block)
      delete_property(name)
      properties << PropertyDeclaration.new(name, value, &block)
    end
def add_property(name, value = nil, &block)
      delete_property(name)
      properties << PropertyDeclaration.new(name, value, &block)
    end
Add a PropertyDeclaration to this mapping

 @param [Symbol] name
 @param [Hash] value ; defaults to nil

 @return [ChildDeclaration]