You can express it formally in terms of a rewrite rule: If Δ is a derivation ending in an expression of the form α(((Qβ)γ)δ)ι, then Δ followed by the term αβι i

You can express it formally in terms of a rewrite rule: If Δ is a derivation ending in an expression of the form α(((Qβ)γ)δ)ι, then Δ followed by the term αβι is a derivation. Writing it as \xyz.x is just a less noisy way of saying that.