# Combining SU(N) multiplets using Young diagrams

I am trying to follow the Particle Data Group's instruction (PDF link) to combine SU(N) multiplets. On page 3, they show an example calculation of SU(3)'s $$\textbf 8\otimes \textbf 8$$.

I understand step (b), i.e. how to add the $$a$$'s, but then for the next step, why is there no diagram like this,

[ ][ ] a a b
[ ]


i.e. a $$(4,1)$$ multiplet of multiplicity $$\textbf{35}$$?