/** Parses:
    *  <term>     ::= { <unop> } <term> | <constant> | <factor> {( <exp-list> )}
    *  <constant> ::= <null> | <int> | <bool>
    * @param token   first token in input stream to be parsed; remainder in Lexer named in.
    */
  private AST parseTerm(Token token) {

    if (token instanceof Op) {
      Op op = (Op) token;
      if (! op.isUnOp()) error(op,"unary operator");
      return new UnOpApp(op, parseTerm(in.readToken()));
    }
    
    if (token instanceof Constant) return (Constant) token;
    AST factor = parseFactor(token);
    Token next = in.peek();
    if (next == LeftParen.ONLY) {
      in.readToken();  // remove next from input stream
      AST[] exps = parseArgs();  // including closing paren
      return new App(factor,exps);
    }
    return factor;
  }