fn impl(rest: List(a), first: a, prev: a) -> #(List(#(a, #(a, a))), a) {
  case rest {
    [] -> #([], first)
    [x] -> #([#(x, #(prev, first))], x)
    [x, y, ..xs] -> {
      let #(next_list, last) = impl([y, ..xs], first, x)
      #([#(x, #(prev, y)), ..next_list], last)
    }
  }
}

pub fn find_neighbours(sites: List(a)) -> List(#(a, #(a, a))) {
  case sites {
    [] -> []
    [x] -> [#(x, #(x, x))]
    [x, y] -> [#(x, #(y, y)), #(y, #(x, x))]
    [x, y, ..xs] -> {
      let #(final, last) = impl([y, ..xs], x, x)
      [#(x, #(last, y)), ..final]
    }
  }
}