The Bell triangle: B(n) counts partitions of an n-element set
B(n) = Σ S(n,k) for k=0..n | EGF: exp(exp(x)−1)
The Bell triangle is built row by row: start each row with the last element of the previous row, then add consecutive pairs. The left edge gives Bell numbers: 1, 1, 2, 5, 15, 52, 203, ...