This happens, which is bad.

[james-d-mitchell]

>>> S = FroidurePin(Transformation([1, 0]))
>>> S.cbegin_idempotents()
input_line_60:6:219: error: 'IteratorPairFirstTraits' is a private member of
      'libsemigroups::FroidurePin<libsemigroups::Transformation<unsigned char>,
      libsemigroups::FroidurePinTraits<libsemigroups::Transformation<unsigned
      char> > >'
  ...char> > >::IteratorPairFirstTraits>) (((libsemigroups::FroidurePin<libse...
                ^
/usr/local/include/libsemigroups/froidure-pin-impl.hpp:1593:24: note: declared private here
  struct FROIDURE_PIN::IteratorPairFirstTraits
                       ^
input_line_60:6:409: error: no member named 'cbegin_idempotents' in
      'libsemigroups::FroidurePin<libsemigroups::Transformation<unsigned char>,
      libsemigroups::FroidurePinTraits<libsemigroups::Transformation<unsigned
      char> > >'
  ...((libsemigroups::FroidurePin<libsemigroups::Transformation<unsigned char>,libsemigroups::FroidurePinTraits<libsemigroups::Transformation<unsigned char> > >*)obj)->c...
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  ^
input_line_60:10:170: error: no member named 'cbegin_idempotents' in
      'libsemigroups::FroidurePin<libsemigroups::Transformation<unsigned char>,
      libsemigroups::FroidurePinTraits<libsemigroups::Transformation<unsigned
      char> > >'
  ...((libsemigroups::FroidurePin<libsemigroups::Transformation<unsigned char>,libsemigroups::FroidurePinTraits<libsemigroups::Transformation<unsigned char> > >*)obj)->c...
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  ^
Error in <TClingCallFunc::make_wrapper>: Failed to compile
  ==== SOURCE BEGIN ====
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wformat-security"
__attribute__((used)) extern "C" void __cf_23(void* obj, int nargs, void** args, void* ret)
{
   if (ret) {
      new (ret) (libsemigroups::detail::ConstIteratorStateless<libsemigroups::FroidurePin<libsemigroups::Transformation<unsigned char>,libsemigroups::FroidurePinTraits<libsemigroups::Transformation<unsigned char> > >::IteratorPairFirstTraits>) (((libsemigroups::FroidurePin<libsemigroups::Transformation<unsigned char>,libsemigroups::FroidurePinTraits<libsemigroups::Transformation<unsigned char> > >*)obj)->cbegin_idempotents());
      return;
   }
   else {
      ((libsemigroups::FroidurePin<libsemigroups::Transformation<unsigned char>,libsemigroups::FroidurePinTraits<libsemigroups::Transformation<unsigned char> > >*)obj)->cbegin_idempotents();
      return;
   }
}
#pragma clang diagnostic pop
  ==== SOURCE END ====
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: libsemigroups::detail::ConstIteratorStateless<libsemigroups::FroidurePin<libsemigroups::Transformation<unsigned char>,libsemigroups::FroidurePinTraits<libsemigroups::Transformation<unsigned char> > >::IteratorPairFirstTraits> libsemigroups::FroidurePin<libsemigroups::Transformation<unsigned char>,libsemigroups::FroidurePinTraits<libsemigroups::Transformation<unsigned char> > >::cbegin_idempotents() =>
    ValueError: nullptr result where temporary expected
>>>