nested congr doesn't work in converter mode

[fpvandoorn]

Description

In the code snippet below, the second congr raises an error, presumably because I'm already "inside" an funext. In my actual case I wanted to rewrite inside a double integral.

example : (∃ x y : ℕ, x = y) = true :=
begin
  conv { to_lhs, congr, funext, congr, }
end