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

A kernel from a measurable space `α` to another measurable space `β` is a measurable function
`κ : α → Measure β`. The measurable space structure on `MeasureTheory.Measure β` is given by
`MeasureTheory.Measure.instMeasurableSpace`. A map `κ : α → MeasureTheory.Measure β` is measurable
iff `∀ s : Set β, MeasurableSet s → Measurable (fun a ↦ κ a s)`.

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
```