#24 Checker should be able to find refinements in jar files

[Gary T. Leavens]

If a -sourcepath element is a jar file, then the checker's
treatment of refinement doesn't find the file. It's
probably treating the jar file as a directory. However,
both for compatability with javac and for easier
installation of JML, it would be very convenient if we
could use source files (e.g., of the JDK) from a jar file
(e.g. src.jar) when making refinements.

The workaround for this is to unjar the src.jar file in the
JDK and put the directory containing that in the
sourcepath.

[Gary T. Leavens]

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This was also reported by a user, so I'm increasing the
priority.