feat(model_theory/basic): Terms, formulas, and definable sets (#11067)
feat(model_theory/basic): Terms, formulas, and definable sets (#11067)

Defines first-order terms, formulas, sentences and theories
Defines the boolean algebra of definable sets
(Several of these definitions are based on those from the flypitch project.)





leanprover-community/mathlib,leanprover-community/mathlib