Initialize alphabet from an iterable of token or subtoken strings.

def _init_alphabet_from_tokens(self, tokens):
        """Initialize alphabet from an iterable of token or subtoken strings."""
        # Include all characters from all tokens in the alphabet to guarantee that
        # any token can be encoded. Additionally, include all escaping
        # characters.
        self._alphabet = {c for token in tokens for c in token}
        self._alphabet |= _ESCAPE_CHARS