_finish(): In delete-a-version clause, use next_dup() instead of
next() to iterate over the currentVersions table with cursor set to
the vid of the version we're deleting.

abortVersion(): When cruising over the currentVersions table (and the
key set to vid of the version we're aborting), use next_dup() instead
of next().  Also, when curvid <> vid, raise StorageSystemError instead
of VersionError.  Same later on when we check curvid <> zero.

commitVersion(): Add sanity checking based on model documentation: if
the source version is the empty string, or src == dest, raise a
VersionCommitError.  Make the same changes we made previously to
abortVersion() to keep track of the oids in a set/dictionary instead
of a list to avoid duplicates.  Other fixes include, writing revid
instead of tid==self._serial to write_moved_object() and adding a
write_discard_version(svid) once the source version's been committed.

commitVersion() now works!

load(): Add another check to avoid finding an object revision in a
committed or aborted version if we can guess that version name.  It
now raises a VersionError if vid==zero and version is true.

Also, although we still raise VersionError later on, elaborate the
actual exception better in the error message.

    checkAbortVersionErrors
    checkModifyAfterAbortVersion
    checkCommitToNonVersion
    checkCommitToOtherVersion
    checkAbortOneVersionCommitTheOther

All are inhibited for Minimal storage.

(a.k.a. pickle pointer) is usually tid but not always!  In fact, it's
specifically not tid when we've aborted a version -- it'll be the
pointer to the pickle of the last non-version revision.  We were
actually setting up lrevid correctly, but writing tid to the metadata
record.  This fix writes lrevid instead.

storage has a failing abortVersion(), so it's mostly a checkpoint.

Not yet tested: commitVersion(), transactionalUndo(), undoLog().

Not yet implemented: history(), pack()

Specific changes:

    - Table txnOids renamed to txnoids everywhere.

    - Define a module global `zero' constant and use (via global)
      where ever appropriate (should be named ZERO but that's ugly).

    - _finish(): Only boost the refcount of objects referenced by the
      pickle when we've actually got a new pickle.  Also, add a note
      about the currentVersions table, which will contain multiple
      copies of vid/oid records.  It's easier to weed them out later
      than to weed them out here.  One of the places you need to weed
      them all out is at the end of this method, in the
      `delete-a-version' branch.

    - abortVersion(): Record oids found by scanning the
      currentVersions table in a set, to account for the possible
      multiple vid/oid records.  Fix some typos.  When calling
      write_nonversion_object() on the CommitLog, the prevrevid ought
      to be the revid of the current object revision, for undo
      purposes.

    - load(): Where we previously had a comment that asked whether we
      should assert that the version is empty (after we've checked
      everything else), now we really check that and raise a
      VersionError if not.

    - loadSerial(): Fix a typo.

    - store(): Inside the old-revision-id-is-true test, if the old vid
      is not zero and it is equal to the new vid, then the non-version
      revision of the object is the old non-version revision (added
      the else: clause).

    - transactionalUndo(): Implemented but not yet tested.

    - undo(): Removed.

    - versionEmpty(): Semantics are to return true if an unknown
      version is given.  As an extra bonus, if version is empty
      (i.e. the non-version) check the serials table.

common operation. ;)  Also, _dostore() automatically pickles the data
argument, since you don't really want anything else.

Lots of new tests:
    checkNonVersionModifiedInVersion()
    checkLoadSerial()
    checkVersionedStoreAndLoad()
    checkVersionedLoadErrors()
    checkConflicts()
    checkVersionLock()
    checkVersionEmpty()
    checkVersions()
    checkAbortVersion()

Skip for the Minimal storage:
    checkVersionedLoadErrors()
    checkVersionLock()
    checkVersionEmpty()
    checkVersions()
    checkAbortVersion()

Override for the Minimal storage (basically check for Unsupported exceptions):
    checkLoadSerial()
    checkVersionedStoreAndLoad()

long to make it more readable.

checkNonVersionStore(): Add some default arguments to make it useful
for checkNonVersionStoreAndLoad().

__init__(), close(): Remove self._references since we sniff the pickle
instead.

_decref() -> _zaprevision(): Also, changed implementation since we
observe that it can't really be shared with the _decref() for pack,
and we're moving to a non-destructive undo instead.  So make this
function (presumably) useful for pack() and cyclic garbage
collection.  We'll see...

undo(): Mark this TBR -- to be removed -- but don't (yet) remove it.

the Minimal and Full storages.

more code sharing.

DBHOME is unused, remove.

given by tpc_begin().

_finish(), _abort(): Fix typos.

TextMixin class.

storages, like transaction aborts and commits, changing objects, etc.
Doesn't test undo, versions, or packing.

"Using not needed variable"