struct strfreq(string x)
{
  res = mkstruct();
  for (i = 0; i < strlen(x); i++) {
    char = tolower(substr(x, i, 1));
    if (!isalpha(char)) continue;
    res = struct_set(res, char, struct_get(res, char) + 1);
  }
  return res;
}

setlocale("");

if (argc < 2) {
  print("not enough arguments\n");
  exit(1);
}

freq = strfreq(argv[1]);
letters = struct_fields(freq);

for (i = 0; i < (int) letters; i++) {
  letter = letters[i];
  print(letter, " occured ", struct_get(freq, letter), " times\n");
}