& Example Ludus implementation of the Sierpinski triangle.
& https://en.m.wikipedia.org/wiki/L-system#Example_5:_Sierpinski_triangle

& The Sierpinski triangle drawn using an L-system
& variables : F G
& constants : + -
& start  : F-G-G
& rules  : (F -> F-G+F+G-F), (G -> GG)
& angle  : 120°
& F and G both mean "draw forward", + means "turn left by angle", and − means "turn right by angle".

& set a length for each segment
let length = 10

& set an angle for all turns, default to 1/3 for triangles
let angle = inv(3)

& define 'G' function from the grammaer
fn g! {
  (0) -> fd!(length)
  (n) -> {
    g!(dec(n))
    g!(dec(n))
  }
}

& define 'F' function from the grammaer
fn f! {
  (0) -> fd!(length)
  (n) -> {
    f!(dec(n))
    rt!(angle)
    g!(dec(n))
    lt!(angle)
    f!(dec(n))
    lt!(angle)
    g!(dec(n))
    rt!(angle)
    f!(dec(n))
  }
}

& this function defines the starting pattern.
& each of the f! and g! calls expand based on the grammar at the top of this example
fn sierp! (times) -> {
  f!(times)
  rt!(angle)
  g!(times)
  rt!(angle)
  g!(times)
}

& call the base function with a number of iterations
& remember, this is exponential! high numbers can freeze your browser.
sierp!(7)