anchor.diff:    let divergence (where: Coord) =
       let cell = Grid.raw_get where
       let is_nonsolid (_, n) = let c = Grid.raw_get n in c.is_not_solid ()
  -    let backward = where.backward_neighbors ()
  -    let outgoing = Seq.filter is_nonsolid backward
  +    let outgoing = where.backward_neighbors ()
                      |> Seq.fold (fun accum (dir, _) ->
                                     let v = Coord.border dir cell.velocity
                                     accum .+ v
  
positive.code:   // verify that pressures look sane.
    let check_pressures markers =
      for m in markers do
        let p = m |> get_pressure |> ((/) (LanguagePrimitives.FloatWithMeasure 1.0))
        if Double.IsNaN p then
          failwith "Invalid pressure."
        if Double.IsNegativeInfinity p then
  
negative.code:     let c = (Constants.h * Constants.fluid_density) / dt
      let b = Seq.map (fun m ->
                         let f = divergence m
                         let result: float<kg/(m*s^2)> = c * f // ensure we have units of pressure.
                         float result // remove pressure units (matrix solver does not support units).
                       ) markers
                       |> Seq.toList
  