code: def addProperty(self, property):
  		
  		if self.has_key(property.id):
  			raise DuplicatePropertyError
  		else:
  			self.data[property.id] = property
  			self.order.append(property)
docs: Adds a 'Property' to this 'Node'. Checks for duplicate properties
  			(illegal), and maintains the property order. Argument:
  			- property : 'Property