Lean-Workbook Syntax 34585: A partial alternating binomial sum

For natural $n,m$, $\sum_{k=0}^{m}(-1)^k\binom{2n+1}{k}=(-1)^m\binom{2n}{m}$.

Source: Lean-Workbook row lean_workbook_plus_34585 (Apache-2.0), original record. This corrected declaration replaces the obsolete finite sum/product binder in with ∈ and restores required imports or namespaces. Ranges, casts, quantifiers, and mathematical expressions are preserved.