Create set of characters that appear in any element in the iterable.

def _generate_alphabet_dict(iterable, reserved_tokens=None):
  """Create set of characters that appear in any element in the iterable."""
  if reserved_tokens is None:
    reserved_tokens = RESERVED_TOKENS
  alphabet = {c for token in iterable for c in token}
  alphabet |= {c for token in reserved_tokens for c in token}
  alphabet |= _ESCAPE_CHARS  # Add escape characters to alphabet set.
  return alphabet