Check whether the character is a letter (as opposed to a diacritic or
	suprasegmental).

	In strict mode return True only if the letter is part of the IPA spec.

def is_letter(char, strict=True):
	"""
	Check whether the character is a letter (as opposed to a diacritic or
	suprasegmental).

	In strict mode return True only if the letter is part of the IPA spec.
	"""
	if (char in chart.consonants) or (char in chart.vowels):
		return True

	if not strict:
		return unicodedata.category(char) in ['Ll', 'Lo', 'Lt', 'Lu']

	return False