The groups facility allows you to both combine groups into larger groups and to exclude groups to reduce the test coverage. This is used to great effect to handle tests for optional components - eg something that exists in a full JDK but not a jre - in its simplest form:

jdk = \
  :jre \
  :needs_jdk

jre = \
  test/ \ # all tests
 -:needs_jdk

needs_jdk = ...

However there are a number of "optional" components leading to a combinatorial explosion of potentially needed groups. Rather than explicitly defining all these groups it would be very convenient to be able to implicitly define a new group on the command-line by selecting various existing groups for includusion and exclusion. E.g.:

jtreg :jdk -x :needs_a -x :needs_b