operator allowed compared closures with each other.
* Implement prophecy variables using the new support for "observations". The
erasure theorem (showing that prophecy variables do not alter program
behavior) can be found in the iris/examples repository, in
`theories/logatom/proph_erasure.rs`.
behavior) can be found [in the iris/examples repository](https://gitlab.mpi-sws.org/iris/examples/blob/3f33781fe6e19cfdb25259c8194d34403f1134d5/theories/logatom/proph_erasure.v).
* heap_lang now uses right-to-left evaluation order. This makes it
significantly easier to write specifications of curried functions.
* heap_lang values are now injected in heap_lang expressions via a specific
