[Agda] How to read constraints?

[Robin Green]

I do not understand the output of the Show Constraints command, and I
cannot find a reference for it.

Here is what it prints:

_403 := λ F' G' H' GF' {d'} {a'} {b'} {c'} h' g' f' GFD' →
  composition h' (λ x → g' (f' x))  |  [record {domain = a; codomain =
  d; fn = λ x → h (g (f x))} = ?0 F G H GF h g f GFD : Function]

What does this mean?

I note that there seems to be no term in common between the left side
of the vertical bar and the right side of it, as all abstracted
variables on the left side are primed.

-- 
Robin