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Ponder's type system is unusual. It is more powerful than the

extended by Haskell. Ponder adds extra recursive 'mu' types

to those of Girard's System F, allowing more general

recursion. Surprisingly, the type system and typeinference algorithm are still not completely understood.

["Ponder and its Type System", J. Fairbairn, TR 31, Cambridge

U Computer Lab, Nov 1982].

[J. Fairbairn, "Design and Implementation of a Simple Typed

Language based on the Lambda-Calculus", Technical Report

No. 75, Computer Laboratory, University of Cambridge, May

1985].

[J. Fairbairn, "A New Type-Checker for a Functional Language",

Technical Report No. 53, Computer Laboratory, University of

Cambridge, 1984].

[J. Fairbairn, "Some Types with Inclusion Properties in

forall, rightarrow, mu", Technical Report No. 171, Computer

Laboratory, University of Cambridge, Jun 1989].

[Valeria C. V. de Paiva, "Subtyping in Ponder (Preliminary

Report)", Technical Report No. 203, Computer Laboratory,

University of Cambridge, Aug 1990].