Step 4 of 6

Review: modifying while iterating

Another classic: changing a container while looping over it.

• erasing index i shifts the next element into position i, and then i++ skips it
• push_back during a range-for can reallocate, invalidating the loop's hidden iterators
• a reference taken before a push_back can dangle afterwards

Your turn: this pull request enforces bans and fills the roster back up with substitutes. Fix it so that every banned player is removed (including when two are adjacent), one substitute is added per removed player, and captain returns the right name. There are three bugs.

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