Check (naively) whether the two tokens can form a diphtong. This would be a
	sequence of vowels of which no more than one is syllabic. Vowel sequences
	connected with a tie bar would already be handled in tokenise_word, so are
	not checked for here.

	Users who want more sophisticated diphtong detection should instead write
	their own function and do something like::

		tokenise(string, diphtong=False, merge=user_func)

	Helper for tokenise(string, ..).

def are_diphtong(tokenA, tokenB):
	"""
	Check (naively) whether the two tokens can form a diphtong. This would be a
	sequence of vowels of which no more than one is syllabic. Vowel sequences
	connected with a tie bar would already be handled in tokenise_word, so are
	not checked for here.

	Users who want more sophisticated diphtong detection should instead write
	their own function and do something like::

		tokenise(string, diphtong=False, merge=user_func)

	Helper for tokenise(string, ..).
	"""
	is_short = lambda token: '◌̯'[1] in token
	subtokens = []

	for char in tokenA+tokenB:
		if ipa.is_vowel(char):
			subtokens.append(char)
		elif ipa.is_diacritic(char) or ipa.is_length(char):
			if subtokens:
				subtokens[-1] += char
			else:
				break
		else:
			break
	else:
		if len([x for x in subtokens if not is_short(x)]) < 2:
			return True

	return False