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let hexchar_to_int = function
    | x when x >= '0' && x <= '9' -> int x - int '0'
    | x when x >= 'A' && x <= 'F' -> int x - int 'A' + 10
    | _ -> 0

let hex_to_int(xs : string) : bigint = 
    let mutable res = 0I in 
    for c in xs do 
        res <- res * 16I + (new bigint(hexchar_to_int c)) 
        done;
    res

//    Console.WriteLine(hex_to_int("A0"));
//    Console.WriteLine(hex_to_int("10000"));
//    Console.WriteLine(hex_to_int("BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB"));