LilyPond Contributor's Guide: 10.8.2 Write regression tests

### 10.8.2 Write regression tests

In order to demonstrate that the code works properly, you will need to write one or more regression tests. These tests are typically ‘`.ly`’ files that are found in ‘`input/regression`’.

Regression tests should be as brief as possible to demonstrate the functionality of the code.

Regression tests should generally cover one issue per test. Several short, single-issue regression tests are preferred to a single, long, multiple-issue regression test.

Use existing regression tests as templates to demonstrate the type of header information that should be included in a regression test.

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