Function signFromAssumedPart

src/compute-engine/library/complex.ts:29–40

(
  ce: ComputeEngine,
  op: Expression,
  part: SubjectPart
)

 * when the operand is a symbol with no value, look up assumed bounds for the
 * corresponding subject (e.g. `im:tau` after `assume(Im(tau) > 0)`) and
 * derive the sign from them (docs/fungrim/FUNGRIM-PLAN-3-ASSUMPTIONS.md §5.1b).
 *
 * Reads the fact index directly (never `ask()`), so it works inside
 * `verify()`. Returns `undefined` when the facts don't entail a sign.
 */
export function signFromAssumedPart(
  ce: ComputeEngine,
  op: Expression,
  part: SubjectPart
): Sign | undefined {
  if (!isSymbol(op) || op.value !== undefined) return undefined;
  // Fast gate: engines with no assumptions do no index work.
  if (!hasAssumptions(ce)) return undefined;
  return signFromBounds(
    getInequalityBoundsFromAssumptions(ce, { symbol: op.symbol, part })