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Proper indentation is very important. Just because the code lines up properly in your editor of choice, does not mean it will line up properly for someone else working on the project. This can be very annoying. These requirements will prevent this.
Tabs should be used to indent all code blocks. Spaces should never be used to indent code blocks. Mixing tabs and spaces results in misaligned code blocks for other developers who prefer different indentation settings. Consider the following code block which is indented with all tabs (shown with and without whitespaces):
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