Help me please to find a solution for the last formula in exercise 12.8. from "Introduction to mathematical logic" by A.Church. Here it is:

Quote:

Prove as theorem of $\displaystyle \mathrm{P_{\mathrm{W}}}$ without making use of the fourth axiom $\displaystyle f\supset p$:

$\displaystyle [p\supset r]\supset[[[p\supset q]\supset r]\supset r]$.