(Reuters) - Activist investor Appaloosa LP responded to proposed concessions from the board of Allergan Plc by saying it had done everything "except what needs to be done" and calling on other investors to take further "disruptive" steps to change the company. The hedge fund, led by billionaire David Tepper, has been pressing Allergan since last year to separate the roles of CEO and chairman or potentially sell or merge the company. Allergan on Friday agreed to split its chairman and chief executive roles, but only at its next scheduled leadership change.