anchor.diff:  
     use bl_constants_module, only: ZERO, THIRD, HALF, ONE, TWO
     use prob_params_module, only: center, problo
  -  use meth_params_module, only: do_rotation, rot_period
  +  use meth_params_module, only: do_rotation, rot_period, rot_period_dot
     use rot_sources_module, only: get_omega, cross_product
     
     implicit none
  
positive.code: 
    dV = dx(1) * dx(2) * dx(3)
    dQtt(:,:) = ZERO
  
    if (do_rotation .eq. 1) then
       omega = get_omega(time)
    else
       omega = ZERO
    endif
  
    ! If we are in a rotating reference frame, then we want
    ! to rotate this by an amount corresponding to the time
    ! that has passed since the beginning of the simulation.
  
negative.code:   ! at large distances. There is some clipping due to material
    ! that leaves or enters the domain boundaries, but this should
    ! average out over time, and will be negligible in magnitude.
  
    ! To get the angle, we integrate omega over the time of the
    ! simulation. Since the time rate of change is linear in the
    ! period, let's work that variable. At some time t the current
    ! period P is given by P = P_0 + Pdot * t. Then:
    !
    ! theta(t) = int( omega(t) * dt )
    !      theta = int( omega(t) dt )
    !            = (2 * pi / P_0) * int( dt / (1 + (dPdt / P_0) * t) )
    !
    ! if dPdt = 0, then theta = 2 * pi * t / P_0 = omega_0 * t, as expected.
    ! if dPdt > 0, then theta = (2 * pi / P_0) * (P_0 / dPdt) * ln| (dPdt / P_0) * t + 1 |
    ! Note that if dPdt << P_0, then we have ln(1 + x) = x, and we again
    ! recover the original expression as expected.
  
    if (do_rotation .eq. 1) then
    
       if (abs(rot_period_dot) > ZERO .and. time > ZERO) then
          theta = get_omega(ZERO) * (rot_period / rot_period_dot) * &
                  log( abs( (rot_period_dot / rot_period) * time + 1 ) )
       else
          theta = get_omega(ZERO) * time
       endif
  
       omega = get_omega(time)
       
    else
  
       omega = ZERO
       theta = ZERO
  
    endif
         
    ! This is the 3D rotation matrix for converting between reference frames.
    ! It is the composition of rotations along the x, y, and z axes. Therefore 
    ! it allows for the case where we are rotating about multiple axes. Normally 
  