Menger curvature and rectifiability in metric spaces

We show that for any metric space X the condition∫X∫X∫Xc(z1,z2,z3)2dH1z1dH1z2dH1z3<∞, where c(z1,z2,z3)c(z1,z2,z3) is the Menger curvature of the triple (z1,z2,z3)(z1,z2,z3) and H1H1 is the 1-dimensional Hausdorff measure on X, guarantees that X is rectifiable.