uncaught exception: error(existence_error(procedure, p/1), top_level/0)

I am trying to teach myself prolog starting with the J. R. Fisher tutorial https://www.cpp.edu/~jrfisher/www/prolog_tutorial/pt_framer.html. I worked through the first few exercises using gprolog with no problems. I exited prolog for a break, but now when I restarted it I find myself unable to define procedures and am getting an uncaught exception:

gprolog
GNU Prolog 1.4.5 (64 bits)
Compiled Jul 14 2018, 16:25:15 with /usr/bin/clang
By Daniel Diaz
Copyright (C) 1999-2018 Daniel Diaz
| ?- p(a).
uncaught exception: error(existence_error(procedure,p/1),top_level/0)

I suspect I exited prolog improperly and there is an invalid state somewhere in a cache. FWIW I am running on a MacBook with OS X Big Sur version 11.3. Are there any suggestions about how to fix this?