old_contents: """
  Base dimensions
  
  
  """
  
  #-----------------------------------------------------------------------------
  # Copyright (c) 2013, yt Development Team.
  #
  # Distributed under the terms of the Modified BSD License.
  #
  # The full license is in the file COPYING.txt, distributed with this software.
  #-----------------------------------------------------------------------------
  
  from sympy import Symbol, sympify, Rational
  
  mass = Symbol("(mass)", positive=True)
  length = Symbol("(length)", positive=True)
  time = Symbol("(time)", positive=True)
  temperature = Symbol("(temperature)", positive=True)
  angle = Symbol("(angle)", positive=True)
  current_mks = Symbol("(current_mks)", positive=True)
  dimensionless = sympify(1)
  
  base_dimensions = [mass, length, time, temperature, angle, current_mks,
                     dimensionless]
  
  #
  # Derived dimensions
  #
  
  rate = 1 / time
  
  velocity     = length / time
  acceleration = length / time**2
  jerk         = length / time**3
  snap         = length / time**4
  crackle      = length / time**5
  pop          = length / time**6
  
  area     = length * length
  volume   = area * length
  momentum = mass * velocity
  force    = mass * acceleration
  energy   = force * length
  power    = energy / time
  flux     = power / area
  specific_flux = flux / rate
  
  # Gaussian electromagnetic units
  charge_cgs  = (energy * length)**Rational(1, 2)  # proper 1/2 power
  current_cgs = charge_cgs / time
  electric_field_cgs = charge_cgs / length**2
  magnetic_field_cgs = electric_field_cgs
  
  # SI electromagnetic units
  charge_mks = current_mks * time
  electric_field_mks = force / charge_mks
  magnetic_field_mks = electric_field_mks / velocity
  
  solid_angle = angle * angle
  
  derived_dimensions = [rate, velocity, acceleration, jerk, snap, crackle, pop, 
                        momentum, force, energy, power, charge_cgs, electric_field_cgs,
                        magnetic_field_cgs, solid_angle, flux, specific_flux, volume,
                        area, current_cgs, charge_mks, electric_field_mks,
                        magnetic_field_mks]
  
  dimensions = base_dimensions + derived_dimensions
  
new_contents: """
  Base dimensions
  
  
  """
  
  #-----------------------------------------------------------------------------
  # Copyright (c) 2013, yt Development Team.
  #
  # Distributed under the terms of the Modified BSD License.
  #
  # The full license is in the file COPYING.txt, distributed with this software.
  #-----------------------------------------------------------------------------
  
  from sympy import Symbol, sympify, Rational
  
  mass = Symbol("(mass)", positive=True)
  length = Symbol("(length)", positive=True)
  time = Symbol("(time)", positive=True)
  temperature = Symbol("(temperature)", positive=True)
  angle = Symbol("(angle)", positive=True)
  current_mks = Symbol("(current_mks)", positive=True)
  dimensionless = sympify(1)
  
  base_dimensions = [mass, length, time, temperature, angle, current_mks,
                     dimensionless]
  
  #
  # Derived dimensions
  #
  
  rate = 1 / time
  
  velocity     = length / time
  acceleration = length / time**2
  jerk         = length / time**3
  snap         = length / time**4
  crackle      = length / time**5
  pop          = length / time**6
  
  area     = length * length
  volume   = area * length
  momentum = mass * velocity
  force    = mass * acceleration
  energy   = force * length
  power    = energy / time
  flux     = power / area
  specific_flux = flux / rate
  
  # Gaussian electromagnetic units
  charge_cgs  = (energy * length)**Rational(1, 2)  # proper 1/2 power
  current_cgs = charge_cgs / time
  electric_field_cgs = charge_cgs / length**2
  magnetic_field_cgs = electric_field_cgs
  
  # SI electromagnetic units
  charge_mks = current_mks * time
  electric_field_mks = force / charge_mks
  magnetic_field_mks = electric_field_mks / velocity
  
  # Since cgs is our default, I'm adding these shortcuts for backwards-compatibility
  charge = charge_cgs
  electric_field = electric_field_cgs
  magnetic_field = magnetic_field_cgs
  
  solid_angle = angle * angle
  
  derived_dimensions = [rate, velocity, acceleration, jerk, snap, crackle, pop, 
                        momentum, force, energy, power, charge_cgs, electric_field_cgs,
                        magnetic_field_cgs, solid_angle, flux, specific_flux, volume,
                        area, current_cgs, charge_mks, electric_field_mks,
                        magnetic_field_mks]
  
  dimensions = base_dimensions + derived_dimensions
  
subject: Add these for backwards-compatibility