Source code for com.ibm.team.filesystem.ide.ui plugin
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Like any other source code that is made available, this is part of the SDK.
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If you want to work with the source code, follow https://rsjazz.wordpress.com/2015/09/30/learning-to-fly-getting-started-with-the-rtc-java-apis/ , perform the extensions workshop at least lab 1, to setup a development environment based on the SDK.