How subsequent alt should be written in sequence diagram?

[arvenil]

Hi, if I understand correctly according to https://mermaid.js.org/syntax/sequenceDiagram.html#alt an alt without else should be written as opt? Then why something like this is possible?

sequenceDiagram
    Alice->>Bob: Hello Bob, how are you?
    alt is sick
        Bob->>Alice: Not so good :(
    end
    opt Extra response
        Bob->>Alice: Thanks for asking
    end

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Let's consider now additional alt to check for headache.

Should it be

sequenceDiagram
    Alice->>Bob: Hello Bob, how are you?
    alt is sick
        Bob->>Alice: Not so good :(
    end
    opt Extra response
        Bob->>Alice: Thanks for asking
    end
    alt Bob has headache
        Alice->>Bob: Take a painkiller
    end

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Or

sequenceDiagram
    Alice->>Bob: Hello Bob, how are you?
    alt is sick
        Bob->>Alice: Not so good :(
        alt Bob has headache
            Alice->>Bob: Take a painkiller
        end
    end

    opt Extra response
        Bob->>Alice: Thanks for asking
    end

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Or

sequenceDiagram
    Alice->>Bob: Hello Bob, how are you?
    alt is sick
        Bob->>Alice: Not so good :(
        alt Bob has headache
            Alice->>Bob: Take a painkiller
        else Bob doesn't have headache
            Alice->>Bob: Get well!
        end
    else is well
        Bob->>Alice: Feeling fresh like a daisy
    end
    opt Extra response
        Bob->>Alice: Thanks for asking
    end

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