Ronen Barzel and Alan H. Barr.
  <b>A Modeling System Based on Dynamic Constraints</b>.
  <i>Computer Graphics</i>, 22(4):179-188, August 1988.
  <A HREF="http://vis.cs.brown.edu/docs/pdf/bib/g/Barzel-1988-MSB.pdf.html">(pdf)</A>
  <A HREF="http://vis.cs.brown.edu/results/bibtex/Barzel-1988-MSB.bib">(bibtex:
  Barzel-1988-MSB)</A>.<br />