///|
/// Mark the outer box as no-slip walls.
pub fn Lattice::set_box_walls(self : Lattice) -> Unit {
  for x in 0.. Unit {
  for x in 0.. Unit {
  for x in 0.. Unit {
  let r2 = radius * radius
  for y in 0.. Unit {
  if x >= 0 && x < self.size.width {
    for y in 0.. Unit {
  let h = (self.size.height - 1).to_double()
  for y in 0.. Unit {
  let y = self.size.height - 1
  for x in 1..<(self.size.width - 1) {
    self.set_solid(x, y, solid=false)
    self.set_cell(x, y, rho~, ux~, uy=0.0)
  }
}