def extract_char_tokens(word_counts):
    """Extracts all single-character tokens from word_counts.
    Args:
      word_counts: list of (string, int) tuples
    Returns:
      set of single-character strings contained within word_counts
    """

    seen_chars = set()
    for word, _ in word_counts:
        for char in word:
            seen_chars.add(char)
    return seen_chars