  We add to the mounting evidence that the topological B model's normalized
holomorphic three-form has integral periods by demonstrating that otherwise the
B2-brane partition function is ill-defined. The resulting Calabi-Yau manifolds
are roughly fixed points of attractor flows. We propose here that any
admissible background for topological strings requires a quantized (twisted)
integrable pure spinor, yielding a quantized (twisted) generalized Calabi-Yau
structure. This proposal would imply in particular that the A model is
consistent only on those Calabi-Yau manifolds that correspond to melting
crystals. When a pure spinor is not quantized, type change occurs on positive
codimension submanifolds. We find that quantized pure spinors in topological
A-model instead change type only when crossing a coisotropic 5-brane. Quantized
generalized Calabi-Yau structures do correspond to twisted K-theory classes,
but some twisted K-theory classes correspond to either zero or to multiple
structures.
