Tag for matrices with sub_matrix function exist and doesn't say for which ranges it is defined. The documentation for this struct was generated from the following file:
mtl::tag::has_sub_matrix Struct Reference -- MTL 4 -- Peter Gottschling and Andrew Lumsdaine
-- Generated on 19 May 2009 by Doxygen 1.5.5 -- Copyright 2007 by the Trustees of Indiana University.