code: function expression(rbp) {
  			var fnLeft;
  
  			oToken = advance();
  			if (!oToken) {
  				error("Expected expression but instead saw end of input", sInput);
  			}
  			fnLeft = mSymbols[oToken.id].nud(oToken, oParser);
  
  			while (iNextToken < aTokens.length) {
  				oToken = current();
  				if (rbp >= (mSymbols[oToken.id].lbp || 0)) {
  					break;
  				}
  				advance();
  				fnLeft = mSymbols[oToken.id].led(oToken, oParser, fnLeft);
  			}
  
  			return fnLeft;
  		}
docs: Parse an expression starting at the current token. Throws an error if there are no more
  tokens and
  
  @param {number} rbp
  a "right binding power"
  @returns {function} The formatter function for the expression