Nuprl also has a well developed X-Windows user interface and allows for the use of clear and concise notations, close to ones used in print.
Proofs are objects which can be viewed, and serve as readable explanations of theorems.
Tactics provide a high-level extendible toolkit for proof development, while the soundness of the system relies only a fixed set of rules. Paul B. Jackson. Nuprl and its use in circuit design. In R.T. Boute V. Stavridou, T.F.Melham, editor, Proceedings of the 1992 International Conference on Theorem Provers in Circuit Design, IFIP Transactions A-10. Paul B. Jackson.