groff ships with Eric Raymon's extended manual for pic, but nothing similar for eqn.
If the AT&T release would permit it, and Gnu would honor AT&T's terms, a Kernighan/Cherry document could be distributed with groff. K&C documents are widely available online. Source text is at https://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/doc/eqn. At the very least a reference should be given in eqn(1). Inexplicably, The Texbook is listed instead. Doug
