Returns whether the given part represents an output variable.

def _is_output(part):
    """ Returns whether the given part represents an output variable. """
    if part[0].lower() == 'o':
        return True
    elif part[0][:2].lower() == 'o:':
        return True
    elif part[0][:2].lower() == 'o.':
        return True
    else:
        return False