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編譯器的虛擬yacc工具

To do a visualization type:
    gram.out < <input_string>


input1:
program test (out);
var x: integer;
begin
  x := 3;
  while (x > 0) do
    x := x - 1
end.

input2:
program test (out);
var max,x: integer;
begin
  max := 44;
  readln(x);
  if x > max then
     x := x - max
  else
     x := max - x;
  writeln(x) 
end.

input3:
program test (out);
var max,x: integer;
  function foo(x : integer) : integer; 
  begin
    if x = 0 then 
      foo:= 1
    else 
     foo := foo(x - 1)*2
  end;

begin
  max := 44;
  readln(x);
  if x > max then
     x := x - max
  else
     x := max - x;
  writeln(foo(x)) 
end.