Finally the Greatest Fibonacci in J!

NB. Generates y Fibonacci numbers starting with 0 1.
NB. If the y argument is boxed, generates
NB. Fibonacci numbers up to the given number.
NB. Tribonacci numbers (and so on) can be generated
NB. as follows: 0 0 1x fibonacci 1e3 or (<3) fibonacci 1e3.
fibonacci =: 3 : 0 "0 1 0
    0x 1x fibonacci y
:
  if. L.x do.
    x=.1x,~0$~<:>x
  end.
  (,a)^:((>y)>#`(a=.+/@((-#x)&{.))@.(L.y))^:_ x
)

NB. Examples:

NB. First 100 Fibonacci numbers
fibonacci 100

NB. Fibonacci numbers up to 100
fibonacci <100

NB. Two ways to do the first 10 Tribonacci numbers
0 0 1x fibonacci 10
(<3) fibonacci 10

NB. The Quadribonacci numbers up to 1e6
(<5) fibonacci <1e6