private bool IsGhostedMultiplicationSign(IExpressionUnit currentExpressionUnit)
        {
            if (previousExpressionUnit == null)
            {
                return(false);
            }

            return(previousExpressionUnit.IsCloseBracket() && currentExpressionUnit.IsNumber() ||
                   previousExpressionUnit.IsCloseBracket() && currentExpressionUnit.IsOpenBracket() ||
                   previousExpressionUnit.IsNumber() && currentExpressionUnit.IsOpenBracket());
        }