code: def link(self, other):
          
          p = other.__pts__
          return self.properties.link(p)
docs: Link (associate) this object with anI{other} properties object
          to create a network of properties.  Links are bidirectional.
          @param other: The object to link.
          @type other: L{Properties}
          @return: self
          @rtype: L{Properties}