John Cowan may have added srfi-144-constants.scm in anticipation of
adding a Chicken-specific implementation.

By the way, I forgot to note that the portable sample implementation
was designed to work in R6RS and R5RS with minimal changes.  Indeed, I did most of the development in Larceny's R5RS mode, where it was convenient to have access to all of the un-exported help procedures, including those that ended up being commented out.