plain tex. beginsection doesnt break line

This is the definition of beginsection from plain.tex:

outerdefbeginsection#1par{vskipz@ plus.3vsizepenalty-250
  vskipz@ plus-.3vsizebigskipvskipparskip
  message{#1}leftline{bf#1}nobreaksmallskipnoindent}

This means that the title is indeed set inside leftline, which doesn't allow line breaks. How can you modify it? Remove leftline but add a local setting of interlinepenalty in order that the paragraph formed by the section title is not split across pages.

catcode`@=11

outerdefbeginsection#1par{%
  vskipz@ plus.3vsize % a generous stretchability
  penalty-250 % a good point to break a page
  vskipz@ plus-.3vsize % but cancel it if no page break is taken at the penalty
  bigskipvskipparskip % separation from the above text if no page break
  message{#1}% log the section title
  begingroup
  interlinepenalty=10000 % no page break allowed in the title if split across lines
  hangindent=parindent hangafter=1 % indent all lines like the first one
  bf#1par
  endgroup
  nobreak % no page break between the title and the following text
  smallskip
  noindent
}

catcode`@=12

vsize=6baselineskip % just to show that no break is taken

beginsection This is a long title that should be split across lines; just
  make it sufficiently long; maybe this long is sufficient

Text of the section

beginsection 
  This is a long title that should be split across lines; just
  make it sufficiently long; maybe this long is sufficient
  This is a long title that should be split across lines; just
  make it sufficiently long; maybe this long is sufficient
  This is a long title that should be split across lines; just
  make it sufficiently long; maybe this long is sufficient

Text of the section

bye