*** [neqn] Error code 1

[bob prohaska]

Recent buildworld attempts have stopped with:

--- neqn ---e 6082)
Making neqn from /usr/src/gnu/usr.bin/groff/src/preproc/eqn/../../../../../../contrib/groff/src/preproc/eqn/neqn.sh
sed: 1: "s/^[1-9].*$/.&/": bad flag in substitute command: 's'
sed: 1: "s|@g@||g": bad flag in substitute command: 's'
*** [neqn] Error code 1

The last few updates haven't touched on the files involved, so perhaps it's
local to my system:
11.0-CURRENT FreeBSD 11.0-CURRENT #94 r299289

The sources are presently at 299460.

Any hint of a workaround would be appreciated!

Thanks for reading,

bob prohaska