Pyramid Technology Corporation 90x RISC-based minicomputer ran OSx, which supported BSD and System V at the same time in parallel universes, and had patented "conditional symbolic links" to support dynamically switching between the two by changing an environment variable.
>Pyramid Technology's OSx Operating System implemented conditional symbolic links which pointed to different locations depending on which universe a program was running in. The universes supported were AT&T's SysV.3 and the Berkeley Software Distribution (BSD 4.3). For example: if the ps command was run in the att universe, then the symbolic link for the directory /bin would point to /.attbin and the program /.attbin/ps would be executed. Whereas if the ps command was run in the ucb universe, then /bin would point to /.ucbbin and /.ucbbin/ps would be executed. Similar Conditional Symbolic Links were also created for other directories such as /lib, /usr/lib, /usr/include.
One of the ways it did this was that filesystem symlinks could reference the environment variable that specified what OS emulation you wanted.
https://en.wikipedia.org/wiki/Domain/OS
https://jim.rees.org/apollo-archive/