The following PL/0 program is fib.0.
module main;
var result:int;
procedure fib(arg:int);
var temp:int;
begin
if arg <= 0 then result := 0; end;
if arg = 1 then result := 1; end;
if arg > 1 then
fib(arg - 1);
temp := result;
fib(arg - 2);
result := result + temp;
end;
end fib;
var n:int;
begin
n := input;
while n <> 0 do
fib(n);
output := result;
n := input;
end;
end main.
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