Sappho survives in fragments — papyrus torn, a word lost to a worm. But the poem is not gone where the words are: the metre fixes the rhythm of the gap even when its letters are lost, and the surviving letters and the grammar narrow what could fill it. The form outlives the loss. Slide the constraints (or tap).
Reconstruction under constraint, on a real principle. A three-syllable lacuna sits at fixed positions of a Sapphic line whose metre is known. Raw, the gap's rhythm could be any of 2³ = 8 long/short sequences — but the metrical template at those positions is FIXED, so the metre alone collapses the rhythm to exactly ONE pattern (its metrical entropy is zero). Add the surviving letters and the Aeolic lexicon and only a handful of words fit that rhythm and those letters. The words may be lost, but their SHAPE is recoverable — the form survives the loss. A fail-loud self-check throws unless the metre reduces the rhythm possibilities from 8 to 1 and the candidate count strictly falls with each constraint.
An illustrative lexicon and a single gap (real papyrology weighs palaeography, sense, parallels and often cannot decide); the candidate words are a demonstration. The 2ⁿ→1 metrical collapse and the monotone narrowing are exact.