text: import simtk.unit as units
  
  from intermol.decorators import accepts_compatible_units
  from intermol.forces.abstract_2_virtual_type import Abstract2VirtualType
  
  
  class TwoVirtualType(Abstract2VirtualType):
      __slots__ = ['a', 'placeholder']
  
      @accepts_compatible_units(None, None, None, 
                                a=units.dimensionless,
                                placeholder=None)
      def __init__(self, bondingtype1, bondingtype2, bondingtype3, 
                   a=0.0 * units.dimensionless,
                   placeholder=False):
          Abstract2VirtualType.__init__(self, bondingtype1, bondingtype2, bondingtype3, placeholder)
          self.a = a
  
  
  class TwoVirtual(TwoVirtualType):
      """
      stub documentation
      """
      def __init__(self, atom1, atom2, atom3, bondingtype1=None, bondingtype2=None, bondingtype3=None, 
                   a=0.0 * units.dimensionless,
                   placeholder=False):
          self.atom1 = atom1
          self.atom2 = atom2
          self.atom3 = atom3
          TwoVirtualType.__init__(self, bondingtype1, bondingtype2, bondingtype3,
                  a=a,
                  placeholder=placeholder)
  