Lean-Workbook Syntax 67809: A finite binomial convolution

$\sum_{k=3}^{51}\binom{k}{3}\binom{52-k}{1}=\binom{53}{5}$.

Source: Lean-Workbook row lean_workbook_plus_67809 (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.