=== results ===

  ok   pl_create
  ok   pl_consult loads a file
  ok   goal succeeds
  ok   goal fails
  ok   consulted predicate runs
  ok   consulted predicate fails
  ok   error sets get_error
  ok   error clears afterwards
  ok   4 solutions: likes(_, _)
  ok   2 solutions: likes(mary, _)
  ok   1 solution: likes(john, mary)
  ok   3 solutions: member/2
  ok   pl_query on a fresh goal
  ok   pl_done releases it
  ok   second pl_create
  ok   assert visible in its own engine
  ok   assert invisible to the other
  ok   first engine survives the second's destroy
  --   gap: string literal in a backtracked goal    fewer than 4 solutions, or an error: the goal's string was freed by pl_query
  --   gap: no-solution vs one-solution after pl_query indistinguishable, both report 1

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