Whenever I&#x27;m out of ideas I always end up going back to the Propagator&#x2F;Logic Simulator chapters of SICP and rewriting them hoping to glean a bit more  about the process and usability of such an abstraction.<p>I usually end up writing a Propagating logic simulator (combining the chapters) and marveling at the working forward&#x2F;backward ripple carry adder.  This time I&#x27;ve derived all the way down to switches which infer unknown values based on their current inputs and used them to successfully create the reversible logic functions required by the Propagator model.<p>Fascinating stuff and highly recommended reading for anybody interested in alternative models of computation.