BalanceCheck            Balance Check for Multiple Covariates in
                        Matched Observational Studies
CrossMST                Covariate balance checking through the minimum
                        spanning tree
CrossNN                 Covariate balance checking through the nearest
                        neighbor graph
getD                    Get intermediate results for CrossNN
getDk                   Get intermediate results for CrossNN under k-NN
getR1R2                 Get intermediate results for CrossMST
smokingDist             The distance matrix of the smoking data
