Until we rose the bark we could not pretend to call it a chase.

How in thunderation could anyone anticipate a result like this?

Gentzen also wrote down inference rules suitable for the intuitionistic propositional and predicate calculi. These rules lack the mirror symmetry of the classical case. But in the 1980s, this symmetry was restored by Girard’s invention of ‘linear logic’ [43].

The handwritten, chalkboard type is called eraser dust.