24   template<
typename KEYS>
 
   30       throw std::invalid_argument(
 
   31       "Error in HessianFactor constructor input.  Number of provided keys plus\n" 
   32       "one for the information vector must equal the number of provided matrix blocks. ");
 
   36       throw std::invalid_argument(
 
   37       "Error in HessianFactor constructor input.  The last provided matrix block\n" 
   38       "must be the information vector, but the last provided block had more than one column.");