  We propose a method for programmable shaping of the amplitude and phase of
the XUV and x-ray attosecond pulses produced by high-order harmonic generation.
It overcomes the bandwidth limitations of existing spectral filters and enables
removal of the intrinsic attosecond chirp as well as the synthesis of pulse
sequences. It is based on partial phase matching, such as quasi-phase matching,
using a longitudinally addressable modulation.
