// public
-void matrix::print_elements(const print_context & c, const std::string & row_start, const std::string & row_end, const std::string & row_sep, const std::string & col_sep) const
+void matrix::print_elements(const print_context & c, const char *row_start, const char *row_end, const char *row_sep, const char *col_sep) const
{
for (unsigned ro=0; ro<row; ++ro) {
c.s << row_start;