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Accessing the Stack Top [TOC]
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The program should now be extended so that a function called `top` can be used
to read the value of the top element of the stack without removing it. In
`main`, the function can be used as follows, for example:

---- CODE(type=abc) ------------------------------------------------------------
fn main()                                                                        
{                                                                                
    push(3);                                                                     
    printf("top = %d\n", top());                                                 
    push(4);                                                                     
    printf("top = %d\n", top());                                                 
    pop();                                                                       
    printf("top = %d\n", top());                                                 
}   
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This should produce the following output:

---- CODE(type=txt) ------------------------------------------------------------
top = 3
top = 4
top = 3
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