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 ECLoperating system. ["Studies in EXtensible Programming Languages", B. Wegbreit, Garland. Pub 1980].