to post messages and comments.

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If you wan to add element in existing list - use nested joins:

ExternalBastionSecurityGroups - List<AWS::EC2::SecurityGroup::Id>
SecurityGroupsList            - List<AWS::EC2::SecurityGroup::Id>
Outputs.BastionSecurityGroup  - AWS::EC2::SecurityGroup::Id

SecurityGroupsList: !Join
  - ','
  - - !GetAtt [ securitygroups, Outputs.BastionSecurityGroup ]
    - !Join [ ',', !Ref ExternalBastionSecurityGroups ]

How to filter intalled on the host certificates by FriendlyName

@powershell "Get-ChildItem -Recurse Cert: | where { $_.FriendlyName -eq 'Starfield Class 2 Certification Authority' -or $_.FriendlyName -eq 'Starfield Services Root Certificate Authority - G2' -or $_.FriendlyName -eq 'Amazon Root CA 1'} | Format-Table -AutoSize | Out-String -Width 2000"