sentences:  Lindström's theorem is perhaps the best known result of what later became known as abstract model theory, the basic notion of which is an abstract logic; the more general notion of an institution was later introduced, which advances from a set-theoretical notion of model to a category-theoretical one.Lindström had previously obtained a similar result in studying first-order logics extended with Lindström quantifiers.Lindström's theorem has been extended to various other systems of logic in particular modal logics by Johan van Benthem 
  and Sebastian Enqvist.