                $result = $d_defs[$attr_key]->validate(
                                $value, $config, $context
                           );
            } else {
                // system never heard of the attribute? DELETE!
                $result = false;
            }

            // put the results into effect
            if ($result === false || $result === null) {
                // this is a generic error message that should replaced
                // with more specific ones when possible
                if ($e) $e->send(E_ERROR, 'AttrValidator: Attribute removed');

                // remove the attribute
                unset($attr[$attr_key]);
            } elseif (is_string($result)) {
                // generally, if a substitution is happening, there
                // was some sort of implicit correction going on. We'll
                // delegate it to the attribute classes to say exactly what.

                // simple substitution
                $attr[$attr_key] = $result;
            } else {
                // nothing happens
            }

            // we'd also want slightly more complicated substitution
            // involving an array as the return value,
            // although we're not sure how colliding attributes would
            // resolve (certain ones would be completely overriden,
            // others would prepend themselves).
        }
The software does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.