Formats a decimal field:

    .. sourcecode::

       1234 ('D') -> 1234
       -1234 ('D6') -> -001234

def format_decimal_field(__, prec, number, locale):
    """Formats a decimal field:

    .. sourcecode::

       1234 ('D') -> 1234
       -1234 ('D6') -> -001234

    """
    prec = 0 if prec is None else int(prec)
    if number < 0:
        prec += 1
    return format(number, u'0%dd' % prec)