A main that does not return an int exits with 0

The first program no longer ends in a 0, and the appendix says what the
compiler knows about main. The unit type is spelled void in the
signatures that wrote unit, which is its value, and print-n-times has a
loop clause for its body.

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Co-Authored-By: eca-agent <git@eca.dev>

_04.Appendices.sz

@@ -67,12 +67,12 @@ When compiled, the frontend rejects this with:
 Type Error: Function 'greet' requires a type signature (: greet ...) at hello.bjo:1:1
 }|
 
-@strong{Why it happens:} Unlike local helper functions defined inside another function, every top-level definition in a module must carry an explicit type signature. The compiler requires this so that module interfaces are self-documenting and can be emitted into the compiled assembly's public metadata without requiring whole-program inference across files. The only exception is @code{main}, which has a fixed signature known to the compiler.
+@strong{Why it happens:} Unlike local helper functions defined inside another function, every top-level definition in a module must carry an explicit type signature. The compiler requires this so that module interfaces are self-documenting and can be emitted into the compiled assembly's public metadata without requiring whole-program inference across files. The only exception is @code{main}: the compiler knows what it takes, the arguments as a @code{(Vec string)}, and what it returns is the exit code when it is an int.
 
 @strong{How to fix it:} Add an explicit signature right before the definition:
 
 @codeblock[#:lang "bjolang"]|{
-(: greet (-> string unit))
+(: greet (-> string void))
 (defun (greet name)
   (println (str "Hello, " name)))
 }|