Standalone .Fa causes the parser to crash

[dspinellis]

A standalone .Fa macro, which is not embedded in .Fo / .Fc, causes the parser to crash with TypeError: Cannot read property 'push' of undefined.

As an example consider the following excerpt from the unlink(2) system call.

The
.Fn unlink
function
removes the link named by
.Fa path
from its directory and decrements the link count of the
file which was referenced by the link.