In the free monoid monad $(T, \eta, \mu)$ in category $Set$:
$T: Set\to Set$ is a endofunctor, is the composition of the free monoid functor $List:Set→Mon$ and the forgetful functor $U:Mon→Set$. $TA$ is the set of ﬁnite lists of elements in set $A$.
$\eta_A: A\to TA$ sends each element $a$ of set $A$ to the corresponding singleton list $[a]$.
What does the multiplication operation $\mu$ of a monad mean? Specifically, what does $\mu_A: T^2A \to TA$ do/mean?