class LetExpr extends Expr {
    
    IdExpr id;
    Expr rhs;
    Expr body;

    static final String TOKEN = "say";

    public String toString( /* LetExpr this */ ) {
        return TOKEN + " " + this.id.toString()
             + " be " + this.rhs.toString()
             + " in " + this.body.toString()
             + " mates";
        }
   
    public Value eval(/* LetExpr this */) {
        Value v = this.rhs.eval();
        Expr ePrime = this.body.subst( this.id, v );
        return ePrime.eval();
        }
      
    
     
      
    public Expr subst( /* LetExpr this, */ IdExpr id, Value v ) {
        return  new LetExpr( this.id, // keep this an id, not a Value
                             this.rhs.subst(id,v),
                             this.body.subst(id,v) );
    }

    public LetExpr( IdExpr _id, Expr _rhs, Expr _body ) {
        this.id = _id;
        this.rhs = _rhs;
        this.body = _body;
        }
        
    @Override
    public boolean equals( /* BinExpr this, */ Object that) {
        if (this==that) {
           return true;
           }
       else if (that==null) {
           return false;
           }
       else if (this.getClass() != that.getClass()) {
           return false;
           }
       else {
           LetExpr thatt = (LetExpr) that;
           return this.id.equals(thatt.id)
               && this.rhs.equals(thatt.rhs)
               && this.body.equals(thatt.body);
           }
    
    }
    
    @Override
    public int hashCode() {
        if (cachedHash == null) {
            int hashSoFar = 0x815F58D4; // fingerprint
            hashSoFar += this.id.hashCode();
            hashSoFar *= 31;
            hashSoFar += this.rhs.hashCode();
            hashSoFar *= 31;
            hashSoFar += this.body.hashCode();
            cachedHash = hashSoFar;
            }
        return cachedHash;
        }
    private Integer cachedHash = null;
    
    }