Last Rule for Connective III: Rule for "⊃": "If φ[n-m] and ψ[n-l] are formulae, φ ⊃n ψ is a formula"

Last formation rule for connective III: the horseshoe:


"If

φ_[n-m]

and

ψ_[n-l]

are formulae,


φ ⊃_n ψ

is a formula."