This is a short one: . Solution:
The way is clear: we have to suppose , and finally see that, in that case, is true. The trick: is always true, whether we suppose or not.
We must use implication introduction, but this needs a hypothesis, and, some lines below, the result of the supposition. Only then we can close the hypothesis.
So after opening it (line 2), we must do something to write down that . Since we already have it written in line 1, we simply put again and justify it with , which means ``I copied this from line 1''. The is for iteration.
We now fulfill the requirements to apply the rule, so we apply it, closing the subdemonstration, and we've ended.