About type()

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I've been going through Stephen C. Baxter's book How To Program Computers. The book is good but the printed code examples are full of typos and other mistakes. Until now I've managed to correct the mistakes with the help of the debugger, but yesterday I came to something I don't understand. It's about the type routine, as he calls it. The code in the example is as follow: 

type string(sequence st) 
    integer ln 
    ln = length(st) 
    for i = 1 to ln do 
        if integer(st[i]) then 
            --ASCII code range 
            if st[i] < 0 and st[i] > 255 then 
                --if one fails, the whole seq fails 
                return 0 
            end if 
        else 
            --if one fails, the whole seq fails 
            return 0 
        end if 
    end for 
    --if you got here, success 
    return 1 
end type 
--Now you can declare variables of the string type 
string fname, lname

This seemed quite cool and was obviously asking to be tested... so I added: 

string flname 
fname = "Awe" 
lname = "some!\n" 
flname = fname & lname 
printf(1, "%s", {flname})

and the output was, as expected... Awesome! So far, so good. However, the example on the book goes on with the following script: 

-- you can also test a variable for string credentials 
if string(seq4) then 
    seq4[1] = 32 
else 
    puts(1, "Not a string.\n") 
end if

And this I couldn't understand. What's this supposed to do?

But my main question about this subject is (as I work a lot with strings): Can this type() things be added to the built-in functions? Thank you.

P.S.: This Stephen C. Baxter is not the famous science fiction author, is it?

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