Theorem mt4i

Modus tollens inference. (Contributed by Wolf Lammen, 12-May-2013.)

Hypotheses
mt4i.1
\( \vdash \chi \)
mt4i.2
\( \vdash \varphi \rightarrow \neg \psi \rightarrow \neg \chi \)
Conclusion
\( \vdash \varphi \rightarrow \psi \)
Proof
mt4i.1
l1
a1i _
\( \vdash \varphi \rightarrow \chi \)
mt4i.2
\( \vdash \varphi \rightarrow \neg \psi \rightarrow \neg \chi \)
mt4d l1 _