def gen_sub_label(lower=False, paren=False):
    """Generates the next letter in the alphabet as a subfig label.
	   Label can be uppercase or lowercase, with optional parentheses.
	"""
    letters = 'A B C D E F G H I J K L M N O P Q R S T U V W X Y Z'
    labels = letters.lower().split(' ') if lower else letters.split(' ')
    for letter in labels:
        yield '(' + letter + ')' if paren else letter
