The phrase “with the insistence of a skilled tactician” suggests that…

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The children were to be driven, as a special treat, to the sands at Jagborough. Nicholas was not to be of the party; he was in disgrace. Only that morning he had refused to eat his wholesome bread-and-milk on the seemingly frivolous ground that there was a frog in it. Older and wiser and better people (his aunt) had told him that there could not possibly be a frog in his bread-and-milk and that he was not to talk nonsense; he continued, nevertheless, to talk what seemed complete nonsense, and described with much detail the coloration and markings of the alleged frog. The dramatic part of the incident was that there really was a frog in Nicholas's basin of bread-and-milk; he had put it there himself, so he felt entitled to know something about it. The sin of taking a frog from the garden and putting it into a bowl of wholesome bread-and-milk was enlarged on at great length, but the fact that stood out clearest in the whole affair, as it presented itself to the mind of Nicholas, was that the older, wiser, and better people had been proved to be profoundly in error in matters about which they had expressed the utmost assurance. "You said there couldn't possibly be a frog in my bread-and-milk; there was a frog in my bread-and-milk," he repeated, with the insistence of a skilled tactician who does not intend to shift from favourable ground.