Janus is a programming language created in 1982 by Christopher Lutz and Howard Derby and Tetsuo Yokoyama and and Robert Glück.
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Janus is a time-reversible programming language written at Caltech in 1982. The operational semantics of the language were formally specified, together with a program inverter and an invertible self-interpreter, in 2007 by Tetsuo Yokoyama and Robert Glück. A Janus inverter and interpreter is made freely available by the TOPPS research group at DIKU. Read more on Wikipedia...
procedure fib from i = 2 do i += 1 x1 <=> x2 x1 -= x2 loop until i = n