        when passing pointers to Python callbacks, the pointee is copied to ensure
          that the Python object never holds a dangling reference. To specify that
          the new Python object should merely contain a copy of a pointer p, the
          user can pass ptr(p) instead of passing p directly. This interface is meant
          to mirror the use of boost::ref(), which can be similarly used to prevent
          copying of referents.
        </p>
<p>
          ptr(p) returns an instance of <a cla