Proof for the meta-theory of HH
Updated the index page of the examples
Support for merlin-mode

http://kiwi.iuwt.fr/~asmanur/blog/merlin/
Typing support for spec-level and.

No support for it at the reasoning level currently.
Reasoning support negative occurrences of & in object sequents.

At runtime, any encounter with a positive & results in the error
message:  "Abella does not support & in clause heads or goals".

Note that this cannot be checked entirely statically as users
can happily do whatever they want using variables of type o
at the reasoning level.
Merge branch 'spec-and' into schemas