import re

TRUTHY = {'yes', 'y', 'on',  'true',  '1'}
FALSEY = {'no',  'n', 'off', 'false', '0'}

def BOOL(s):
    """
    Convert boolean-like strings to `bool` values.  The strings ``'yes'``,
    ``'y'``, ``'on'``, ``'true'``, and ``'1'`` are converted to `True`, and the
    strings ``'no'``, ``'n'``, ``'off'``, ``'false'``, and ``'0'`` are
    converted to `False`.  The conversion is case-insensitive and ignores
    leading & trailing whitespace.  Any value that cannot be converted to a
    `bool` results in a `ValueError`.

    :param string s: a boolean-like string to convert to a `bool`
    :rtype: bool
    :raises ValueError: if ``s`` is not one of the values listed above
    """
    b = s.strip().lower()
    if b in TRUTHY:
        return True
    elif b in FALSEY:
        return False
    else:
        raise ValueError('invalid boolean: ' + repr(s))

def lower(s):
    """
    .. versionadded:: 0.2.0

    Convert ``s`` to lowercase by calling its :meth:`~str.lower()` method if it
    has one; otherwise, return ``s`` unchanged
    """
    try:
        return s.lower()
    except (TypeError, AttributeError):
        return s

def unfold(s):
    r"""
    .. versionadded:: 0.2.0

    Remove folding whitespace from a string by converting line breaks (and any
    whitespace adjacent to line breaks) to a single space and removing leading
    & trailing whitespace.

    >>> unfold('This is a \n folded string.\n')
    'This is a folded string.'

    :param string s: a string to unfold
    :rtype: string
    """
    return re.sub(r'[ \t]*[\r\n][ \t\r\n]*', ' ', s).strip(' ')