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