public async Task Indent_WriteIndentation_Unindent_WriteIndentAsync()
        {
            // arrange
            var stringBuilder = new StringBuilder();
            var writer        = new DocumentWriter(stringBuilder);

            // act
            writer.Indent();
            await writer.WriteIndentationAsync();

            await writer.WriteLineAsync();

            writer.Unindent();
            await writer.WriteIndentationAsync();

            // assert
            Assert.Equal(
                "  " + writer.NewLine + string.Empty,
                stringBuilder.ToString());
        }
        public async Task WriteIndentationAsync()
        {
            // arrange
            var stringBuilder = new StringBuilder();
            var writer        = new DocumentWriter(stringBuilder);

            // act
            await writer.WriteIndentationAsync();

            // assert
            Assert.Equal(string.Empty, stringBuilder.ToString());
        }