module ext_letting_go_and_taking_hold
  depends on letting_go_and_taking_hold

  function declarations
    fluents 
      basic
        is_at : spatial_entity * spatial_entity -> booleans
 
  axioms     
    impossible occurs(X) if instance(X, let_go_of),
                            object(X, O),
                            agent(X, A),
                            -is_at(A, O).

    impossible occurs(X) if instance(X, take_hold),
                            object(X, O),
                            agent(X, A),
                            -is_at(O, A).