Either a vector of halting test functions (or function names) with the signature halting_test(condition, covariates, thresh) (for the meaning of the parameters see match_groups); or it may be a list of list(test = halting_test, cond = subset_of_conditions, cov = variable_selector, thresh) fields. All fields can be left out except test, and test need not be named if it is the first item in the list. The subset_of_conditions can be names of the conditions to match (a character vector or a factor). The variable_selector can be a logical vector with as many items as there will be columns in covariates (recommended), or a vector of integer covariate column indices. Each halting_test is then only applied to the specified subset of conditions and variables of the covariate matrix, with the specified threshold; when a value is not specified the defaults are used. Note that ordering the functions does not change the behavior, but can make the execution of the combined function faster, as the later ones are often evaluated only if the criteria for the earlier ones is met.