code: function next_line() {
  
  // Put the next line of source in source_line. If the line contains tabs,
  // replace them with spaces and give a warning. Also warn if the line contains
  // unsafe characters or is too damn long.
  
              let at;
              column = 0;
              line += 1;
              source_line = lines[line];
              if (source_line !== undefined) {
                  at = source_line.search(rx_tab);
                  if (at >= 0) {
                      if (!option.white) {
                          warn_at("use_spaces", line, at + 1);
                      }
                      source_line = source_line.replace(rx_tab, " ");
                  }
                  at = source_line.search(rx_unsafe);
                  if (at >= 0) {
                      warn_at(
                          "unsafe",
                          line,
                          column + at,
                          "U+" + source_line.charCodeAt(at).toString(16)
                      );
                  }
                  if (option.maxlen && option.maxlen < source_line.length) {
                      warn_at("too_long", line, source_line.length);
                  } else if (!option.white && source_line.slice(-1) === " ") {
                      warn_at(
                          "unexpected_trailing_space",
                          line,
                          source_line.length - 1
                      );
                  }
              }
              return source_line;
          }
docs: the current line source string