old_contents: """This module provides the Quantity class, for tracking of an expression quantity.
  
  Copyright 2020--2021 Michael Hayes, UCECE
  
  """
  
  class Quantity(object):
  
      quantity = ''    
      is_power = False
      is_voltage = False
      is_current = False
      is_impedance = False
      is_admittance = False
      is_transfer = False
      is_immitance = False
      is_always_causal = False
      is_undefined = False
      is_voltagesquared = False
      is_currentsquared = False
      is_impedancesquared = False
      is_admittancesquared = False
      is_ratio = False
      part = ''
  
  
  class UndefinedQuantity(Quantity):
  
      quantity = 'undefined'        
      is_undefined = True    
  
  
new_contents: """This module provides the Quantity class, for tracking of an expression quantity.
  
  Copyright 2020--2021 Michael Hayes, UCECE
  
  """
  
  class Quantity(object):
      """This is the base class for VoltageMixin, etc.  An Lcapy quantity is
      not a true quantity but a collection of related quantities, for
      example, voltage (with units V) and voltage spectral density (with units V/Hz)."""
  
      quantity = ''    
      is_power = False
      is_voltage = False
      is_current = False
      is_impedance = False
      is_admittance = False
      is_transfer = False
      is_immitance = False
      is_always_causal = False
      is_undefined = False
      is_voltagesquared = False
      is_currentsquared = False
      is_impedancesquared = False
      is_admittancesquared = False
      is_ratio = False
      part = ''
  
  
  class UndefinedQuantity(Quantity):
  
      quantity = 'undefined'        
      is_undefined = True
  
subject: Add note on Quantity