[Agda] invert pattern matching

[Martin Escardo]

What does Agda mean when it says

"
Refusing to invert pattern matching of transport because the
maximum depth (50) has been reached. Most likely this means you
have an unsatisfiable constraint, but it could also mean that you
need to increase the maximum depth using the flag
--inversion-max-depth=N
"

What does it mean to "invert pattern matching of transport"?

(I like this error message.)

Thanks,
Martin