fact: Definition De_Morgan_nor := λ"nor" λ"f" "f" @ (λ"a" "nor" @ λ"AZ" λ"BZ" "AZ" @ "a") @ (λ"b" "nor" @ λ"AZ" λ"BZ" "BZ" @ "b").
imports: ClassicalRealizability.Tactics ClassicalRealizability.BasicResults ClassicalRealizability.Subtyping