docs: function next_line() {
              var at;
              if (line >= lines.length) {
                  return false;
              }
              character = 1;
              source_row = lines[line];
              line += 1;
              at = source_row.search(/ \t/);
              if (at >= 0) {
                  warn_at('mixed', line, at + 1);
              }
              source_row = source_row.replace(/\t/g, tab);
              at = source_row.search(cx);
              if (at >= 0) {
                  warn_at('unsafe', line, at);
              }
              if (option.maxlen && option.maxlen < source_row.length) {
                  warn_at('too_long', line, source_row.length);
              }
              return true;
          }