Extensible Language One. An extensible language by B. Wegbreit of Harvard ca 1974. EL1 is internally somewhat Lisp-like, but fully typed with records and pointers. The external syntax is ALGOL-like and extensible, supporting user-defined data structures, control structures and operations. The parser is table-driven, with a modifiable set of productions. Used as the basis for the ECL operating system.

["Studies in Extensible Programming Languages", B. Wegbreit, Garland. Pub 1980].