///|
/// Export rho/ux/uy/speed/solid fields as CSV text.
pub fn Lattice::to_csv(self : Lattice) -> String {
  let b = StringBuilder(size_hint=self.size.width * self.size.height * 40)
  b.write_string("x,y,rho,ux,uy,speed,solid\n")
  for y in 0.. String {
  let ramp = " .:-=+*#%@"
  let b = StringBuilder(size_hint=(self.size.width + 1) * self.size.height)
  let stable = self.stability()
  let denom = if stable.max_speed <= 0.0 { 1.0 } else { stable.max_speed }
  for y in 0.. Option::unwrap)
      }
    }
    b.write_char('\n')
  }
  b.to_string()
}