“…considered by Woitalla in [72], who proved that the algebra M * (O + (2U ⊕ 4A 1 )) at the top is freely generated in weights 4, 4, 6, 6, 8, 10, 12. The bottom of the tower is the famous Igusa algebra [38] of Siegel modular forms of degree two and even weight which is freely generated in weights 4, 6, 10, 12.…”