el1

from The Free On-line Dictionary of Computing (8 July 2008)

EL1

   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].