
This folder contains a proof of the simple fact that an arithmetic progression
is an infinite set from a stronger property of sets stating that:

  a set X which has the finite extension property is infinite

where X is said to have the finite extension property if:

  for every finite subset Y of X there is a finite subset Z of X s.t. Z is a
  strict superset of Y

This proof is formalized in second-order logic and makes heavy use of set
quantifiers.
