William Ackman's Pershing Square Capital Management said it had filed a lawsuit seeking confirmation that its request to hold a special shareholder meeting of class="mandelbrot_refrag"> Allergan Inc would not trigger Allergan's poison pill takeover defense. _0"> Ackman, who owns nearly 10 percent of Allergan, last week filed documents with the Securities and Exchange Commission to call a shareholder meeting to elect new directors to the company's board. The Botox maker has rejected a $53 billion joint offer from Ackman and Canadian drugmaker class="mandelbrot_refrag"> Valeant Pharmaceuticals International . An Allergan spokeswoman declined to comment immediately. Allergan adopted the one-year shareholder rights plan on April 22, the day Valeant and Ackman made the offer, saying it needed time to consider takeover proposals. true Allergan's shareholder rights plan, or poison pill, will be triggered if a person or gro