[Groff] Vertical position not affected by .sp request

[Anton Shepelev]

Hello again,

One  of my macros does not work when called first in
the document, before any text has been  output.  The
problem  is caused by the vertial position registers
(.d and nl) not being affected by the  .sp  request.
Here is an example:

.tm Initial vert. pos.:                 \n[.d]
.sp 40u
.tm Vert. pos. after .sp 40:            \n[.d]
.sp 40u
.tm Vert. pos. after another .sp 40:    \n[.d]
Some text
.sp 40u
.tm Vert. pos. after .sp 40 after text: \n[.d]

If  this is the correct behavour, how to work around
it?

Thanks in advance,
Anton