14. Metre is a satisfiability question, not a comparison

Context

CMUdict records lexical stress, not metrical stress. On "Shall I compare thee to a summer's day" it marks to and shall as stressed, because those are the citation forms. A checker comparing dictionary stress against an iambic pattern would reject Shakespeare's most famous line.

The fact that resolves it: polysyllabic words have fixed internal stress — compare is 01 wherever it appears — while monosyllables take whatever stress the line gives them.

Decision

A line scans when some assignment fits: polysyllabic patterns are fixed, monosyllables are free, secondary stress is free, and every pronunciation CMUdict lists is tried. Because free syllables are independent, this is a linear scan per pronunciation combination rather than a search over stress.

Pronunciation variants matter and are searched. Shakespeare needs the three-syllable tem-per-ate, which CMUdict lists second because the compressed modern form comes first.

Consequences

Metre is checked exactly rather than approximately — no tolerance parameter, no apology. What is checked is strict stress, and that is a narrower claim than it may appear: real English verse routinely uses substitutions, and this checker accepts none of them. Sonnet 18's second line fails, correctly, because temperate ends on an unstressed syllable where strict iambic pentameter wants a stress.

The catalogue definitions say so. A future substitutions parameter could admit the inverted first foot and the weak ending, but encoding all of scansion's licences is a research problem, and claiming to have done it would be the overclaiming this project has avoided throughout.

Two further limits worth stating: CMUdict is modern, so temperate no longer rhymes with date as it did for Shakespeare; and archaic contractions such as o'er are absent entirely, where the checker raises rather than guessing.