You can use the code from these repositories to experiment, test, build, create patches, issue pull requests, etc.
featuremodel/org.eclipse.featuremodel.metamodel.git
featuremodel/org.eclipse.featuremodel.evaluation.git
featuremodel/org.eclipse.featuremodel.editor.diagram.git
featuremodel/org.eclipse.featuremodel.pvbridge.git
featuremodel/org.eclipse.featuremodel.editor.xtext.git
featuremodel/org.eclipse.featuremodel.releng.git