[Pyparsing] Required Class?

[Russell D. <Rus...@as...>]

I have some text I'm parsing with headers and then data. The
header/data groups can be in any order, aba, aabbba, acbd, bdacaa, etc
(Where the letter represents a header type followed by its data). So
overall, I have something like:

a = a_header + a_data
b = b_header + b_data
c = c_header + c_data
d = d_header + d_data

ZeroOrMore(a | b | c | d) + EndTag

Now, if a_header exists, but a_data is malformed, I get an error that
it expected EndTag. For easier debugging, I'd like the error to make
more sense, so:

a = a_header + Required(a_data) where:

class Required(ParseElementEnhance):
    def __init__(self, exprs):
        super(Required, self).__init__(exprs)
    def parseImpl(self, instring, loc, doActions=True):
        try:
            loc, tokens = self.expr._parse(instring, loc, doActions,
callPreParse=False)
        except ParseException as e:
            raise ParseFatalException(e)
        return loc, tokens

Is this the best way to go about this goal?

Is there instead a better way of saying any number of any of these
expressions in any order?

If at least one (or exactly one) 'c' expression is required, is there
a way of denoting that?