Function stepOf

src/compute-engine/boxed-expression/rules.ts:1059–1060

(value: Expression)

      // longer reaches the outer binder's index, and an index colliding with
      // a library constant resolves to the CONSTANT — `Σ_i Σ_j x·i` under
      // `x → 2` answered `8i`, the imaginary unit, instead of `12`
      // (`docs/SCOPING-MODEL.md`, "Rebuilding a scoped node").
      // `scopeForRebuild` withholds the reuse when the rule renamed the binding
      // site, which would otherwise declare the new name into the original
      // expression's live scope.