Hi Stephan, The immediate problem is that you should be using the Mutable_LvaluePropertyMapConcept instead of LvaluePropertyMapConcept. However, I think there may also be a problem with the concept design. Comparing to the iterator concepts, we always have the Mutable version of the concept be a refinement of the not-necessarily mutable version. I think it would be appropriate to change LvaluePropertyMapConcept to require the reference to be convertible to the const&, but not require same type. Cheers, Jeremy __________________________________ Jeremy Siek <siek@cs.colorado.edu> Visiting Assistant Professor Department of Computer Science University of Colorado at Boulder