I found several old copies of tmac.s lying around in my files,
presumably modified for various special purposes.
They all contain not only QP-QE, but also an undocumented QS,
and a never-tested flag set by QP and unset by LP and PP.
It appears that QS was expected to be used when QE is not
immediately followed by another paragraph macro. The hanging
flag suggests that the logic was never fully worked out.
.de QP
.ti \\n(.iu
.RT
.if \\n(1T .sp \\n(PDu
.ne 1.1
.nr QP 1
.in +\\n(QIu
.ll -\\n(QIu
.ti \\n(.iu
..
.de QS
.br
.LP
.in +\\n(QIu
.ll -\\n(QIu
..
.de QE
.br
.ll +\\n(QIu
.in -\\n(QIu
.LP
..
