more robust build_hierarchy: support Resources.empty / Sessions.Structure.empty (required for Build_Job.print_log);
removed unused imports_hierarchy;
theory Quickcheck_Nesting_Example
imports Quickcheck_Nesting
begin
datatype x = X "x list"
lemma "X a = X b"
quickcheck[exhaustive, size = 4, expect = counterexample]
oops
end