[Groff] refer bug?

[Robert D. Goulding]

It should be documented, at least, that the bibliography command must come
*last* in a .R1 .R2 pairing.  So that

.R1
sort A+D+
bibliography mybooks.ref
.R2

is correct, but

.R1
bibliography mybooks.ref
sort A+D+
.R2

not only is incorrect, but dumps core.  I haven't tested it, but I think
this happens when any command follows a bibliography command.  Can this be
fixed so that it emits an error message instead?

Robert.