  After introducing a d=10 pure spinor $\lambda^\alpha$, the Virasoro
constraint $\partial x^m \partial x_m =0$ can be replaced by the twistor-like
constraint $\partial x^m (\gamma_m \lambda)_\alpha=0$. Quantizing this
twistor-like constraint leads to the pure spinor formalism for the superstring
where the fermionic superspace variables $\theta^\alpha$ and their conjugate
momenta come from the ghosts and antighosts of the twistor-like constraint.
