Branch :
quote fn (x) { x }
quote fn (x, _y) { x }
quote fn ((x | _y)) { x }
quote fn {
(()) { :error }
((x | _y)) { x }
(_) { :error }
}
fn (x) { x }
fn (x, _y) { x }
fn ((x | _y)) { x }
fn {
(()) { :error }
((x | _y)) { x }
(_) { :error }
}