Tri-bimaximal neutrino mixing, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:msub><mml:mi>A</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:math> and the modular symmetry