svoid setOneLineFileInfoField(File f, S field, S value) {
  File infoFile = associatedInfosFile(f);
  lines = loadTextFile(infoFile);
  S newLine = field + ": " + value;
  S line = firstStartingWithIC(lines, field + ": ");
  if (eq(line, newLine)) ret;
  saveTextFile(infoFile, lines(replaceOrAdd(lines, line, newLine)));
}