Peirce's Law

Context 1
Natural deduction.

Definition
$$((p \Rightarrow q) \Rightarrow p) \vdash p$$

Proof
This is proved by the tableau method:

Comment
A non-obvious result that has the same strength as the classical Law of the Excluded Middle.

Context 2
Classical Propositional Calculus

Definition
$$((p \Rightarrow q) \Rightarrow p) \Rightarrow p$$

Reference

 * Peirce's Law @ MyWikiBiz

To Do. Translate logical graph proof into local format.