Add a field to a record. Example:

        marc.add('020', a='0201657880', z='0201802398')

def add(self, tag, data=None, indicators=None, **kwargs):
        """
        Add a field to a record. Example:

        marc.add('020', a='0201657880', z='0201802398')
        """
        if tag.startswith("00") and (data is None or data == "") and not self.strict:
            return

        if data:
            if indicators:
                raise ValueError(Record.E_NO_INDICATORS)
            if not tag.startswith('00'):
                raise ValueError(Record.E_NO_DATA)
        else:
            if tag.startswith('00'):
                raise ValueError(Record.E_EMPTY)

        if tag.startswith('00') and kwargs:
            raise ValueError(Record.E_NO_SUBFIELDS)

        if indicators is None:
            indicators = [' ', ' ']
        if isinstance(indicators, basestring):
            if len(indicators) == 2:
                indicators = [indicators[0], indicators[1]]
            else:
                raise ValueError(Record.E_INVALID_INDICATOR)

        if data:  # == control field (001 -- 009)
            field = pymarc.Field(tag, data=data)
        else:     # == non-control field (010 -- 999)
            if 'subfields' in kwargs:
                sfs = kwargs['subfields']
                subfields = [v for sl in [(a, b) for a, b in zip(sfs[::2], sfs[1::2]) if b] for v in sl]
            else:
                subfields = []
                for key, value in kwargs.items():
                    if value is None:
                        continue
                    key = key.replace('_', '')
                    if isinstance(value, basestring):
                        if value == "":
                            continue
                        subfields += [key, value]
                    elif isinstance(value, collections.Iterable):
                        for val in value:
                            if not isinstance(val, basestring):
                                raise ValueError('subfield values must be strings')
                            if val:
                                subfields += [key, val]
                    else:
                        raise ValueError('subfield values must be strings')

            if not any(subfields[1::2]):
                if self.strict:
                    raise ValueError('none of the subfields contains a value')
                else:
                    return

            field = pymarc.Field(tag, indicators, subfields=subfields)
        self.add_field(field)