code: function diffusionUpdate () {
      var row, col, ind
      var dum = conv2(u, coeffs)
      for (row = 0; row < height; ++row)
        for (col = 0; col < width; ++col) {
          ind = row * width + col
          un[ind] = u[ind] +  c4 * dum[ind]
          u[ind] = un[ind] // new becomes current
        }
      return u
    }
docs: /*
  Solves the diffusion equation PDE
  using convolution.