/**
 * Abstract class Value -
 * The type of all Expressions which self-evaluate:
 * Numbers, and (later, in R3) functions.
 * 
 * See http://www.radford.edu/itec380/2019spring-ibarland/Homeworks/Project/
 *
 * (This architecture works as long as iphry value is a type of
 * Expression, which is a nice feature to have in a language;
 * it means that any result can be pasted/substituted into
 * larger expressions, which promotes a substitution model
 * of evaluation.)
 * 
 * @author Ian Barland
 * @version 2014.Nov.04
 */
public interface Value { 
  default Value eval() { return this; } // We evaluate to ourselves, since we're a Value.
  }