class IdExpr extends Expr {
    String name; // The name of the Q2 identifier
    
    public IdExpr( String _name ) { this.name = _name; }
    
    public String toString() { return name; }
    
    public Value eval() {
        throw new RuntimeException("no such var: " + name );
        }
      
    public Expr subst( /* IdExpr this */ IdExpr id, Value v ) {
        return (this.name.equals(id.name)) ? v : this;
        }

    @Override
    public boolean equals( /* IdExpr this, */ Object that) {
        if (this==that) {
            return true;
            }
        else if (that==null) {
            return false;
            }
       else if (this.getClass() != that.getClass()) {
            return false;
            }
       else {
            IdExpr thatt = (IdExpr) that;
            return this.name.equals(thatt.name);
          }
    
    }
    
  @Override
  public int hashCode() {
    if (cachedHash == null) {
      int hashSoFar = 0x815F58D4; // fingerprint
      hashSoFar += this.name.hashCode();
      cachedHash = hashSoFar;
      }
    return cachedHash;
    }
  private Integer cachedHash = null; 
  }