@Nullable
    private static PsiMethodCallExpression tryExtractCombinedToString(PsiMethodCallExpression afterLoopAppend,
                                                                      List<PsiElement> refs) {
      if (refs.size() == 1 && afterLoopAppend == null) { // case like return sb.append(postfix).toString();
        PsiMethodCallExpression usage = ExpressionUtils.getCallForQualifier((PsiExpression)refs.get(0));
        if (APPEND.test(usage)) {
          PsiMethodCallExpression nextCall = ExpressionUtils.getCallForQualifier(usage);
          if (nextCall != null && "toString".equals(nextCall.getMethodExpression().getReferenceName())) {
            return usage;
          }
        }
      }
      return null;
    }
@Nullable
    private static PsiMethodCallExpression tryExtractCombinedToString(PsiMethodCallExpression afterLoopAppend,
                                                                      List<PsiElement> refs)