🚀 go-pugleaf

I wonder whether anyone has come across this problem before, or knows the
cause of it.

The following ditroff input:

.de W
.nr f \\$1
.tm f is \\nf
.if \\nf==0 .tm f should be 0, is \\nf
.if \\nf==1 .tm f should be 1, is \\nf
.if \\nf==2 .tm f should be 2, is \\nf
..
.W 0
.W 1
.W 2

when run with

	troff foobar > /dev/null
	
gives this output (honestly)

f is 0
f should be 1, is 0
f is 1
f should be 0, is 1
f is 2
f should be 0, is 2

I know it looks the wrong way round, but it's really true (on my troff).
It works as expected with my nroff.

I tried a few other varieties, and it seems to be only the single letter
register f that mucks everything up.

James.