module ext_unobstructing_and_obstructing
  depends on unobstructing_and_obstructing

  function declarations
    fluents 
      basic 
        is_at : entity * spatial_entity -> booleans

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