code: function isReverseSignContext(stream) {
  	const start = stream.pos;
  	let ch, inCtx = true;
  
  	while (!isSoF(stream)) {
  		stream.backUp(1);
  		ch = stream.peek();
  
  		if (isWhiteSpace(ch)) {
  			continue;
  		}
  
  		inCtx = ch === LEFT_PARENTHESIS || isOperator(ch);
  		break;
  	}
  
  	stream.pos = start;
  	return inCtx;
  }
docs: Check if current stream state is in sign (e.g. positive or negative) context
  for reverse parsing
  @param  {StreamReader} stream
  @return {Boolean}