Multiple file versions - more li

[jbn]

    Apollo has a nice approach in their Domain Software Enginering Environment.
It is possible to have files with multiple versions; there are SCCS-like
utilities for maintaining them, the versions are maintained with backward
differences (the latest version is present in its entirity and old versions
exist as differences from the latest one) and the operating system has
mechanisms for reading the latest version of such files and preventing 
programs that don't know how to update them from changing them.