We present a principle for introducing new types in type theory which generalises strictly positive indexed inductive data types. In this new principle a set A is denned inductively simultaneously with an ^4-indexed set B, which is also defined inductively. Compared to indexed inductive definitions, the novelty is that the index set A is generated inductively simultaneously with B. In other words, we mutually define two inductive sets, of which one depends on the other. Instances of this principle have previously been used in order to formalise type theory inside type theory. However the consistency of the framework used (the theorem prover Agda) is not so clear, as it allows the definition of a universe containing a code for itself. We give an axiomati-sation of the new principle in such a way that the resulting type theory is consistent, which we prove by constructing a set-theoretic model.
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