RosettaCodeData/Task/Fibonacci-sequence/Bracmat/fibonacci-sequence-2.bracmat

(fib=
  last i this new
.   !arg:<2
  |   0:?last:?i
    & 1:?this
    &   whl
      ' ( !i+1:<!arg:?i
        & !last+!this:?new
        & !this:?last
        & !new:?this
        )
    & !this
)