CALLBACK vs CALLBACK!

The document on callback functions shows them specified two ways:

example: make routine! [
    c [callback! [int int return: [int]]]
    return: [int]
] test-lib "test"

and

example: make routine! [
    c [callback [int int return: [int]]]
    return: [int]
] test-lib "test"

Obviously, both cannot be correct.

Also, rarely do we use a word with ! for non-datatypes, although in this case it is "kind of" a datatype.

I need your input on which name you prefer. Thanks.

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