2604.01741 Gargoyle: A Ugly-But-Rigorous Construction of a Borel Set That Is Not F-sigma
We describe Gargoyle, A detailed, fully verified exposition of a specific Borel set in [0,1] that is provably not F-sigma, written to be instructive rather than elegant.. Textbook proofs that there exist Borel sets which are not F-sigma typically appeal to abstract cardinality or Baire-category arguments, leaving the student without a concrete example to carry in memory.