[isabelle-dev] NEWS: support for multiple session root directories / files

[Makarius]

*** General ***

* Session root directories may be specified multiple times: each
accessible ROOT file is processed only once. This facilitates
specification of $ISABELLE_HOME_USER/ROOTS or command-line options like
-d or -D for "isabelle build" and "isabelle jedit". Example:

  isabelle build -D '~~/src/ZF'


This refers to Isabelle/006deaf5c3dc. The key point is that ROOT files
are only accounted once (according to canonical_file), thus the
directory specifications may be quite liberal. The above example also
shows that selection via directories becomes more useful in general, and
we can probably discontunue some existing session group tags, e.g. "ZF"
or even "AFP".


	Makarius