We analyze the onset of incommensurabilities around the VBS point of the S=1
bilinear-biquadratic model. We propose a simple effective field theory which is
capable of reproducing all known properties of the commensurate-incommensurate
transition at the disorder point θvbs. Moreover, the theory
predicts another special point θdisp, distinct from the VBS
point, where the Haldane gap behaves singularly. The ground state energy
density is an analytic function of the model parameters everywhere, thus we do
not have phase transitions in the conventional sense.Comment: 8 pages, 2 figures, to appear in PR