  Weak-lensing distortions of the cosmic-microwave-background (CMB) temperature
and polarization patterns can reveal important clues to the intervening
large-scale structure. The effect of lensing is to deflect the primary
temperature and polarization signal to slightly different locations on the sky.
Deflections due to density fluctuations, gradient-type for the gradient of the
projected gravitational potential, give a direct measure of the mass
distribution. Curl-type deflections can be induced by, for example, a
primordial background of gravitational waves from inflation or by second-order
effects related to lensing by density perturbations. Whereas gradient-type
deflections are expected to dominate, we show that curl-type deflections can
provide a useful test of systematics and serve to indicate the presence of
confusing secondary and foreground non-Gaussian signals.
