declare positively

See: avow
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References in classic literature ?
About four o'clock, Joe descried some object standing out against the vast background of sand, and soon was able to declare positively that there were two palm-trees at no great distance.
He had certainly entered the train alone, and both the guard and attendant were ready to declare positively that no person could have been concealed in it.
Who was he to declare positively that presidential candidate Grace Poe's statement of being a natural-born Filipino was false?