// Last edited on 2013-01-19 07:25:44 by stolfilocal

  // The files "field_{NN}.inc" should define a macro
  // called "field" with parameters {xp,yp,zp,tt,term}
  // where {xp,yp,zp} are the space coordinates,
  // {tt} is time, and {term} is an integer selecting
  // a particular term of the Taylor series of the 
  // flow about the origin.

  //   0   full non-linear flow
  //   1   affine flow part (degree <= 1)
  //   2   residual flow terms (degree >= 2)
  //   3   constant flow part (degree 0)
  //   4   linear flow part (degree 1)
  //   5   rotation flow term (degree 1 antisymmetric) 
  //   6   strain flow term (degree 1 symmetric)  
  //   7   expansion flow term (degree 1 symmetric isotropic)  
  //   8   shear flow term (degree 1 symmetric traceless) 
  //   9   relative flow (full minus constant)

  // The procedure {field} should return the selected 
  // velocity vector {vp} at point {xp,yp,zp} and time {tt}.
  // For 2D flows, {zp} will be 0 and, and {vp} should have 
  // {vp.z == 0}.
  
  // Since movies start at {tt=0} and end at {tt=1},
  // the velocity field should be periodic in {tt} with period 1.
  // It may be independent of {tt} (i.e. the flow may be steady-state).

 // The files "field_{NN}.inc" should also define the scalar
 // {field_max} with the estimated maximum magnitude of the velocity,
 // for arrow length scaling purposes.