in_array on objects with circular references
public function addField ($name, ifaceNode $field)
{
    // Prevent the same field being added multiple times
    if (!in_array ($field, $this -> fields))
    {
        $this -> fields [$name] = $field;
        $field -> setParent ($this);
    }
    else
    {
        throw new InvalidArgumentException ('This field cannot be added to this group');
    }
    return ($this);
}

if (!in_array ($field, $this -> fields, true))