PDP-8 Engineering and Scientific Interpreter eXtended.

PDP-8 Engineering and Scientific Interpreter eXtended
 ESI-X was the first stored program scientific interpreter
 for the PDP-8.
 ESI was designed by Dave Waks on 8/3/66 and was vaguely based on
 RAND's JOSS.
 In 1967 Bob Supnik extended the program to support 8KW of memory,
 and this became the "ESI-extended" or ESI-X
 FOCAL was eventually the successor of ESI-X.