21. Homotopy type theory works on resolving this problem. 22. A prototypical example is intuitionistic type theory , which retains ramification but discards impredicativity. 23. Attempts to formalize the notion of metavariable result in some kind of type theory . 24. The field is an area of active research, as demonstrated by homotopy type theory . 25. Here " simple " indicates that the underlying type theory is simple, not dependent. 26. Martin-L�f's first draft article on type theory dates back to 1971. 27. Epigram exploits the propositions as types principle, and is based on intuitionistic type theory . 28. This is called polymorphism in type theory . 29. In type theory , enumerated types are often regarded as tagged unions of unit types. 30. In the ramified type theory of PM all objects are elements of various disjoint ramified types.
