How to define a "composite type" with the "group" constructor?

Question

I am trying to define a composite type using the "group" compositor (see Elisp Manual, section "Composite Types"). Previously, I managed to define a composite type with the "list" constructor.

The Elisp manual tells that these constructors ("group" and "list") are similar. I get a weird result in the "Customize interface" and an error message displayed in the echo area.

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Sample code

(defcustom group '(1 2 3)
    "here, give a description of the user option."
    :type '(group integer integer integer))

I noticed the message "custom-variable-value-create: Bad format" in the "Messages" buffer.

The manual should be clarified. Someone had the same issue one decade ago: "No doc for `group' in custom :type".

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