Property sets

• Every grove is constructed using a grove plan
• Grove plan is selection of classes and properties from property set
• Each node in a grove belongs to a class defined in the grove plan
• Some properties are intrinsic - automatically part of every property set
• Non-intrinsic properties of a node are defined in the property set for the node's class
• Property set declares allowed data type for value of properties
• Property set can declare condition for node to have a value for a particular property; when condition not satisfied, node will have a "null" value