module ext_perception
  depends on perception

  sort declarations
    intangible_entity :: entity
    conceptual_entity :: intangible_entity

  function declarations
    fluents
      basic 
        is_known : entity -> booleans
        known_to : entity * tangible_entity -> booleans

    statics
      basic
        information_content : conceptual_entity * thing -> booleans

  axioms
    known_to(X, Y) if is_known(X).
    -known_to(X, Y) if -is_known(X).

    impossible occurs(X) if instance(X, recognize),
                            agent(X, A),
                            object(X, O),
                            information_content(I, O),
                            -known_to(I, A).