warn-on-use Makefile.am: $$p

[Sam Steingold]

modules/warn-on-use has this:

Makefile.am:
BUILT_SOURCES += warn-on-use.h
# The warn-on-use.h that gets inserted into generated .h files is the same as
# build-aux/warn-on-use.h, except that it has the copyright header cut off.
warn-on-use.h: $(build_aux)/warn-on-use.h
        $(AM_V_GEN)rm -f $...@-t $@ && \
        sed -n -e '/^.ifndef/,$$p' \
          < $(build_aux)/warn-on-use.h \
          > $...@-t && \
        mv $...@-t $@
MOSTLYCLEANFILES += warn-on-use.h warn-on-use.h-t


what is "$$p"?