Modified the front-end to allow per-branch [mutable] keywords.
Test for new error message.
Added 3 new tests for facts concerning per-branch [mutable]. One test fails.
Fixed the (internal?) fact printer to print "affine" when nothing more
specific is known.
Add two new tests.
Fixed one place where the flavor was incorrectly computed.
Updated mlist.mz to make MNil immutable and MCons mutable.
Updated the test suite.
Added TODO file.
Fixed an apparent omission in Resugar.
Like mlist, bucket can use an immutable BNil.
Noted a BUG in mutableTreeMap.mz (when Empty is made immutable).
Added mlist::freeze.
Added a basic version of "snapshotable lists" in tests/snapshot.mz.