I was going to say, 30 years ago, the 1st edition of the actual Plan 9 wasn't yet released (not until 1992), much less a clone/port of its shell. But it seems that Byron Rakitzis wrote his Unix clone of `rc` in 1991 before Plan 9 was even out! He based it on Tom Duff's paper which described Plan 9's shell.
(Plan 9's `rc` was originally written for 10th edition Unix, and would later get ported back to Unix as part of Russ Cox's plan9port in 2003.)
(Plan 9's `rc` was originally written for 10th edition Unix, and would later get ported back to Unix as part of Russ Cox's plan9port in 2003.)