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#### Detailed Description of If-then-else Format

For lines common to both files, `diff` uses the unchanged line group format. For each hunk of differences in the merged output format, if the hunk contains only lines from the first file, `diff` uses the old line group format; if the hunk contains only lines from the second file, `diff` uses the new group format; otherwise, `diff` uses the changed group format.

The old, new, and unchanged line formats specify the output format of lines from the first file, lines from the second file, and lines common to both files, respectively.

The option `--ifdef=name` is equivalent to the following sequence of options using shell syntax:

    --old-group-format='#ifndef name
    %<#endif /* ! name */
    ' \
    --new-group-format='#ifdef name
    %>#endif /* name */
    ' \
    --unchanged-group-format='%=' \
    --changed-group-format='#ifndef name
    %<#else /* name */
    %>#endif /* name */
    '

You should carefully check the `diff` output for proper nesting. For example, when using the `-D name` or `--ifdef=name` option, you should check that if the differing lines contain any of the C preprocessor directives `#ifdef`, `#ifndef`, `#else`, `#elif`, or `#endif`, they are nested properly and match. If they don't, you must make corrections manually. It is a good idea to carefully check the resulting code anyway to make sure that it really does what you want it to; depending on how the input files were produced, the output might contain duplicate or otherwise incorrect code.

The `patch` `-D name` option behaves like the `diff` `-D name` option, except it operates on a file and a diff to produce a merged file.