Principle of Commutation
Theorem
Formulation 1
$p \implies \paren {q \implies r} \dashv \vdash q \implies \paren {p \implies r}$
Formulation 2
$\vdash \paren {p \implies \paren {q \implies r} } \iff \paren {q \implies \paren {p \implies r} }$