promise solemnly

See: pledge
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References in classic literature ?
I promise solemnly I 'll never say a word to a mortal creature.
He had only to say that he had lost it at cards or something of that sort, and at the same time promise solemnly not to miss a single lecture for three months on end.
Perfect certainty in these matters may forever be beyond mortal ken, but could we not ask every member of the clergy, at his or her induction, to promise solemnly to report statistical information faithfully and precisely?