The runtime of the callcc version builds on code by R. Kent Dybvig,
Simon L. Peyton Jones, and Amr Sabry, as presented in:

    Dyvbig, R. K., Jones, S. P., & Sabry, A. (2007).
    A monadic framework for delimited continuations.
    Journal of Functional Programming, 17(6), 687-730.

The regular expression implementation is by Dorai Sitaram and
obtained from:

    <https://github.com/ds26gte/pregexp/blob/master/pregexp.scm>