A contraction of "Henderson Escher", it's an implementation of Peter Henderson's functional language for the description of M. C. Escher's Square Limit as described in

Henderson, Peter. Functional Geometry. Proceedings of 1982 ACM Sym- posium on Lisp and Functional Programming, 179-187, ACM, (1982)

and

Henderson, Peter (2002) Functional Geometry Higher-Order and Symbolic Computation, 15, (4), pp. 349-365.