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.
fibonacci =: 3 : 0 "0 1 0
  0x 1x fibonacci y
:
  (,a)^:((>y)>#`(a=.+/@((-#x)&{.))@.(L.y))^:_ x
)

NB. Examples:

NB. Generate the first 20 Fibonacci numbers:
fibonacci 20

NB. Generate the first 20 Tribonacci numbers:
0 0 1x fibonacci 20

NB. Generate the Fibonacci numbers up to 1e8:
fibonacci <1e8