From Roger Hui's An Implementation of J.

One summer weekend in 1989, Arthur Whitney visited Ken Iverson at Kiln Farm and produced  on one page and in one afternoon  an interpreter fragment on the AT&T 3B1 computer. I studied this interpreter for about a week for its organization and programming style; and on Sunday, August 27, 1989, at about four o'clock in the afternoon, wrote the first line of code that became the implementation described in this document.

Arthur's one-page interpreter fragment is as follows: