[u, v, w, y]

# field_name (arg_name=arg (, pair)* ) = expression

c = 1.0 - y

fsd () = sqrt (sqr(ddx(c)) + sqr(ddy(c)) + sqr(ddz(c)))

nx = - ddx(c) / fsd
ny = - ddy(c) / fsd
nz = - ddz(c) / fsd

divn (a=1, b=kill, xlow=128, xupp=320) = ddx(nx) + ddy(ny) + ddz(nz)

avg { c, fsd, divn }