C++ Tutorial => Unary Folds

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Example

Unary folds are used to fold parameter packs over a specific operator. There are 2 kinds of unary folds:

Unary Left Fold (... op pack) which expands as follows:
```
((Pack1 op Pack2) op ...) op PackN
```
Unary Right Fold (pack op ...) which expands as follows:
```
Pack1 op (... (Pack(N-1) op PackN)) 
```

Here is an example

template<typename... Ts>
int sum(Ts... args)
{
    return (... + args); //Unary left fold
    //return (args + ...); //Unary right fold

    // The two are equivalent if the operator is associative.
    // For +, ((1+2)+3) (left fold) == (1+(2+3)) (right fold)
    // For -, ((1-2)-3) (left fold) != (1-(2-3)) (right fold)
}

int result = sum(1, 2, 3); //  6

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