public function createInstanceFor($constraintName)
    {
        if (!isset($this->constraintMap[$constraintName])) {
            throw new InvalidArgumentException('Unknown constraint ' . $constraintName);
        }

        if (!isset($this->instanceCache[$constraintName])) {
            $this->instanceCache[$constraintName] = new $this->constraintMap[$constraintName]($this);
        }

        return clone $this->instanceCache[$constraintName];
    }