"link" href="debug_mode_design.html#debug_mode.design.methods.safe_iter" title="Safe Iterators">safe
  iterators</a> that reference sequences of this type and wrap the
  iterators provided by the release-mode base class. The debugging
  container will use only the safe iterators within its own interface
  (therefore requiring the user to use safe iterators, although this
  does not change correct user code) and will communicate with the
  release-mode base class with only the underlying, unsafe,
  release-m