message: elle.multi_index: syntactic sugar for equal_range
diff: #include <boost/multi_index_container.hpp>
  +#include <elle/range.hh>
  +
  namespace elle
  {
  /// Whether \a e is in \a c.
  @@ -11,6 +13,22 @@ namespace elle
  {
  return c.find(e) != c.end();
  }
  +
  + /// The range of values with equal key.
  + template <typename... Args, typename T>
  + auto
  + equal_range(boost::multi_index::detail::hashed_index<Args...> const& c, T const& e)
  + {
  + return as_range(c.equal_range(e));
  + }
  +
  + /// The range of values with equal key.
  + template <typename... Args, typename T>
  + auto
  + equal_range(boost::multi_index::multi_index_container<Args...> const& c, T const& e)
  + {
  + return as_range(c.equal_range(e));
  + }
  }
  namespace boost
  