Accessing the terminating null byte in a string

[adnan]

As per "Nim in Action", strings are lists of char`s terminated by `'0' (page 
29). In the code below, how can I access that null byte?
    
    
    let s = "Abcd"
    echo s[s.len()] == '\0'
    # Error: unhandled exception: index 4 not in 0 .. 3 [IndexError]
    
    
    Run