@unpublished{H88:Calculus,
  title = "A Calculus for the Derivation of {C-MOS} Switching Circuits",
  author = "C. A. R. Hoare",
  year = "1988",
  month = "April -- October",
  note = "Two drafts; appeared as `A Theory for the Derivation of Combinational {C-MOS} Circuit Designs', \textit{Theoretical Computer Science} 90:235--251, 1991, doi 10.1016/0304-3975(91)90309-P",
}