Email-запись: catena-Poly[[diaqua­cadmium(II)]bis­(μ-2,2-dimethyl­butane­dioato-κ(4) O,O′:O′′,O′′′)[diaqua­cadmium(II)]-μ-1,4-bis­(3-pyridylmeth­yl)piperazine-κ(2) N (3):N (3′)]