groff(7) doesn't tell what the hyphenation modes are.
It directs you to info for the "gory details".
But info doesn't tell everything either. It omits .hy 0·
Mode bits 2, 4, and 8 turn off certain hyphenations.
A literal reading of info would have .hy 0 turn them on.
Since groff(7) is otherwise so complete, I'd love to see
it fixed. That can be done succinctly, and more accurately
than info, as follows
.hy same as .hy 1
.hy 0 turn hyphenation off
.hy N turn hyphenation on, modified by some bits of N:
2 don't hyphenate across page boundaries
4 don't hyphenate first two letters of a word
8 don't hyphenate last two letters of a word
Doug
