public ComparisonOptions withCheckerMutator(
     UnaryFunction<List<ColumnChecker>, List<ColumnChecker>> f) {
   ComparisonOptions out = new ComparisonOptions(this);
   out.mutator = f;
   return out;
 }
 public ComparisonOptions withIgnoreMD() {
   ComparisonOptions out = new ComparisonOptions(this);
   out.ignoreMD = true;
   return out;
 }