we call
`next` and do nothing with the return value. Second, we call `next` to get the
value we want to put in the `query` field of `Config`. If `next` returns a
`Some`, we use a `match` to extract the value. If it returns `None`, it means
not enough arguments were given and we return early with an `Err` value. We do
the same thing for the `filename` value.
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<p><code>env::args</code>の戻り値の1番目の値は、プログラム名であることを思い出してください。それは無視し、
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