Proof General interface for Isabelle.

This includes a prover-side PGIP abstraction layer for passing
interface configuration, control commands and display messages to
Proof General.

  pgip_types.ML    -- the datatypes in PGIP and their manipulation
  pgip_input.ML	   -- commands sent to the prover
  pgip_output.ML   -- commands the prover sends out
  pgip_markup.ML   -- markup for proof script documents
  pgip.ML	   -- union of the above
  pgip_tests.ML    -- some basic testing of the API

The code constructs some marshalling datatypes for reading and writing
XML which conforms to the PGIP schema, interfacing with SML types and
some basic types from the Isabelle platform (i.e. URLs, XML).  This
part of the code is intended to be useful for reuse or porting
elsewhere, so it should have minimal dependency on Isabelle and be
written readably.  Some languages have tools for making type-safe
XML<->native datatype translations from a schema (e.g. HaXML for
Haskell) which would be useful here.

The Isabelle specific configuration is in these files:

  pgip_isabelle.ML	  - configure part of PGIP supported by Isabelle
  parsing.ML		  - parsing routines to add PGIP markup to scripts
  preferences.ML	  - user preferences
  proof_general_pgip.ML   - the main connection point with Isabelle, including
			    the PGIP processing loop.

For the full PGIP schema and an explanation of it, see:

   http://proofgeneral.inf.ed.ac.uk/kit
   http://proofgeneral.inf.ed.ac.uk/wiki/Main/PGIP

David Aspinall, Dec. 2006.
$Id$
