def set_pkg_verif_code(self, doc, code):
self.assert_package_exists()<EOL>if not self.package_verif_set:<EOL><INDENT>self.package_verif_set = True<EOL>doc.package.verif_code = code<EOL><DEDENT>else:<EOL><INDENT>raise CardinalityError('<STR_LIT>')<EOL><DEDENT>
Sets the package verification code, if not already set.
        code - A string.
        Raises CardinalityError if already defined.
        Raises OrderError if no package previously defined.