def set_principal_credit_string(self, credit_string=None):
if credit_string is None:<EOL><INDENT>raise NullArgument()<EOL><DEDENT>metadata = Metadata(**settings.METADATA['<STR_LIT>'])<EOL>if metadata.is_read_only():<EOL><INDENT>raise NoAccess()<EOL><DEDENT>if self._is_valid_input(credit_string, metadata, array=False):<EOL><INDENT>self._my_map['<STR_LIT>']['<STR_LIT:text>'] = credit_string<EOL><DEDENT>else:<EOL><INDENT>raise InvalidArgument()<EOL><DEDENT>
Sets the principal credit string.

        :param credit_string: the new credit string
        :type credit_string: ``string``
        :raise: ``InvalidArgument`` -- ``credit_string`` is invalid
        :raise: ``NoAccess`` -- ``Metadata.isReadOnly()`` is ``true``
        :raise: ``NullArgument`` -- ``credit_string`` is ``null``

        *compliance: mandatory -- This method must be implemented.*