A measurable space is a space equipped with a σ-algebra.

A stochastic, sequential algorithm.
At each round, it sees an observation in `𝓞`, then takes an action in `𝓐`, and finally receives
feedback in `𝓨`. The action is a random function of the past rounds and the current observation.

The reverse image of a measurable space under a function. `comap f m` contains the sets
`s : Set α` such that `s` is the `f`-preimage of a measurable set in `β`.

`inferInstance` synthesizes a value of any target type by typeclass
inference. This function has the same type signature as the identity
function, but the square brackets on the `[i : α]` argument means that it will
attempt to construct this argument by typeclass inference. (This will fail if
`α` is not a `class`.) Example:
```
#check (inferInstance : Inhabited Nat) -- Inhabited Nat

def foo : Inhabited (Nat × Nat) :=
  inferInstance

example : foo.default = (default, default) :=
  rfl
```