authentication/LegacyAuthQuickstart/AuthenticationExample/EmailViewController.m [43:62]:
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          FIRMultiFactorResolver *resolver = error.userInfo[FIRAuthErrorUserInfoMultiFactorResolverKey];
          NSMutableString *displayNameString = [NSMutableString string];
          for (FIRMultiFactorInfo *tmpFactorInfo in resolver.hints) {
            [displayNameString appendString:tmpFactorInfo.displayName];
            [displayNameString appendString:@" "];
          }
          [self showTextInputPromptWithMessage:[NSString stringWithFormat:@"Select factor to sign in\n%@", displayNameString]
                               completionBlock:^(BOOL userPressedOK, NSString *_Nullable displayName) {
           FIRPhoneMultiFactorInfo* selectedHint;
           for (FIRMultiFactorInfo *tmpFactorInfo in resolver.hints) {
             if ([displayName isEqualToString:tmpFactorInfo.displayName]) {
               selectedHint = (FIRPhoneMultiFactorInfo *)tmpFactorInfo;
             }
           }
           [FIRPhoneAuthProvider.provider
            verifyPhoneNumberWithMultiFactorInfo:selectedHint
            UIDelegate:nil
            multiFactorSession:resolver.session
            completion:^(NSString * _Nullable verificationID, NSError * _Nullable error) {
              if (error) {
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authentication/LegacyAuthQuickstart/AuthenticationExample/MainViewController.m [141:160]:
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            FIRMultiFactorResolver *resolver = error.userInfo[FIRAuthErrorUserInfoMultiFactorResolverKey];
            NSMutableString *displayNameString = [NSMutableString string];
            for (FIRMultiFactorInfo *tmpFactorInfo in resolver.hints) {
              [displayNameString appendString:tmpFactorInfo.displayName];
              [displayNameString appendString:@" "];
            }
            [self showTextInputPromptWithMessage:[NSString stringWithFormat:@"Select factor to sign in\n%@", displayNameString]
                                 completionBlock:^(BOOL userPressedOK, NSString *_Nullable displayName) {
             FIRPhoneMultiFactorInfo* selectedHint;
             for (FIRMultiFactorInfo *tmpFactorInfo in resolver.hints) {
               if ([displayName isEqualToString:tmpFactorInfo.displayName]) {
                 selectedHint = (FIRPhoneMultiFactorInfo *)tmpFactorInfo;
               }
             }
             [FIRPhoneAuthProvider.provider
              verifyPhoneNumberWithMultiFactorInfo:selectedHint
              UIDelegate:nil
              multiFactorSession:resolver.session
              completion:^(NSString * _Nullable verificationID, NSError * _Nullable error) {
                if (error) {
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