When an observation can not be explained because of the lack of necessary clauses, candidate clauses that must be used to explain the observation are generated (the dotted line pointed by the finger).
Next, in the knowledge base, clauses which are similar to them are selected (the pink clause).
Then, the candidate clauses are transformed to plausible clauses (the blue clause) by referring to the selected clauses (the pink clause) in the knowledge base to explain the observation.

NB: The above moving picture has some errata. ``prove'' should be ``explain''.