fullmatch can break up a large matching problem into smaller
subproblems (for example, using strata defined by exactMatch).
This function lists the subproblems in a match and list whether at least on
treated unit was matched in subproblem. Subproblems that have no matched
treated units are said to have "failed."
subproblemSuccess(x)A named logical vector indicating either success or failure for each subproblem.
The result of fullmatch or pairmatch.