§ Mnemonic for why eta is unit:

• Remember that given an adjunction $F \vdash G$, the unit of the adjunction is $\eta: 1 \to GF$.
• We use the symbol eta because it's yunit, and eta is y in greek (which is why the vim digraph for eta is C-k y*)
• $\eta$ is unit, since when you flip it, you get $\mu$, which is $\mu$-ltiplication (multiplication). Hence $\eta$ is the unit for the multiplication to form a monoidal structure for the monad.