disentangle Isabelle repository "isab" vs. ISAC repository "isa":
isa/ISABELLE_VERSION specifies the required Isabelle version, for the current point of history of "isa";
isa/README.md explains how to work with the two repositories, including "isa" component setup for the Isabelle clone;
3 test/Tools/isac/ADDTESTS/session-get_theory