eqn(7) on OpenBSD
The text 334705b4ae23, as OpenBSD 7.9, OpenBSD 7.8, OpenBSD 7.7, OpenBSD 7.6, OpenBSD 7.5, OpenBSD 7.4, OpenBSD 7.3, OpenBSD 7.2, OpenBSD 7.1, OpenBSD 7.0, OpenBSD 6.9, OpenBSD 6.8, OpenBSD 6.7 shipped it.
eqn language reference for mandoc