  Quasi-Monte Carlo cubature methods often sample the integrand using Sobol'
(or other digital) sequences to obtain higher accuracy than IID sampling. An
important question is how to conservatively estimate the error of a digital
sequence cubature so that the sampling can be terminated when the desired
tolerance is reached. We propose an error bound based on the discrete Walsh
coefficients of the integrand and use this error bound to construct an adaptive
digital sequence cubature algorithm. The error bound and the corresponding
algorithm are guaranteed to work for integrands whose true Walsh coefficients
satisfy certain cone conditions. Intuitively, these cone conditions imply that
the ordered Walsh coefficients do not dip down for a long stretch and then jump
back up. An upper bound on the cost of our new algorithm is given in terms of
the \emph{unknown} decay rate of the Walsh coefficients.
