See <https://bitbucket.org/cowan/r7rs-wg1-infra/src/1add92fc4182bb598a7946d0738c4a5c69787530/CoreAsn1.md>, the  binary version, and <https://bitbucket.org/cowan/r7rs-wg1-infra/src/default/CoreSexps.md>, the text version.  I gave you a bit more than you asked for, but not overwhelmingly so, I think.  There remains the issue that there is no interoperable S-expression representation of true, false, or null, so they'll have to be chosen: Python's sexpdata library makes the reader specify them, which is pretty ugly.

 

John Cowan          http://vrici.lojban.org/~cowan        xxxxxx@ccil.org
People go through the bother of Christmas because Christmas helps them
to understand why they go through the bother of living out their lives
the rest of the year. For one brief instant, we see human society as it
should and could be, a world in which business has become the exchanging
of presents and in which nothing is important except the happiness and
well-being of the ultimate consumer.  --Northrop Frye (1948)