old_contents: class Inducer:
      """
      The abstract base class for all field inducers.
  
      This class is just the placeholder for the root of the inducer tree; it is
      used by Field to do some preliminary argument checking. It defines no
      methods on its own.
      """
      pass
  
  
  class BInducer:
      """
      A magnetic (B) field inducer.
  
      Like inducer, this class is more or less a placeholder. It does define the
      method bfield_strength which must be implemented by all subclasses.
      """
  
      def bfield_at(self, P):
          """Calculate the magnetic field vector at a point."""
          raise NotImplementedError
  
  
  class Field:
      """
      The abstract base class for all fields.
  
      All fields are callable objects; the return value is a vector denoting the
      field strength and direction at a point. Calculation of this vector must be
      implemented in every concrete subclass. It is typically the superposition
      of fields for every inducer in the field.
  
      Fields maintain a list of inducers called ducs.
      """
  
      def __init__(self):
          self.ducs = []
  
      def add_inducer(self, duc):
          """Add an inducer to this field."""
          self.ducs += [duc]
  
      def draw(self, x, y, z):
          """Draw the field vector at the given point."""
          v = self(x, y, z)
  
      def __call__(self, x, y, z): raise NotImplementedError
  
  class BField(Field):
      """
      A magnetic (B) field.
      """
  
      def __call__(self, x, y, z):
          return sum(duc.bfield_at(x, y, z) for duc in self.ducs)
  
new_contents: class Inducer:
      """
      The abstract base class for all field inducers.
  
      This class is just the placeholder for the root of the inducer tree; it is
      used by Field to do some preliminary argument checking. It defines no
      methods on its own.
      """
      pass
  
  
  class BInducer:
      """
      A magnetic (B) field inducer.
  
      Like inducer, this class is more or less a placeholder. It does define the
      method bfield_strength which must be implemented by all subclasses.
      """
  
      def bfield_at(self, P):
          """Calculate the magnetic field vector at a point."""
          raise NotImplementedError
  
  
  class Field:
      """
      The abstract base class for all fields.
  
      All fields are callable objects; the return value is a vector denoting the
      field strength and direction at a point. Calculation of this vector must be
      implemented in every concrete subclass. It is typically the superposition
      of fields for every inducer in the field.
  
      Fields maintain a list of inducers called ducs.
      """
  
      def __init__(self):
          self.ducs = []
  
      def add_inducer(self, duc):
          """Add an inducer to this field."""
          self.ducs += [duc]
  
      def draw(self, P):
          """Draw the field vector at the given point."""
          v = self(P)
  
      def __call__(self, P): raise NotImplementedError
  
  class BField(Field):
      """
      A magnetic (B) field.
      """
  
      def __call__(self, P):
          return sum(duc.bfield_at(P) for duc in self.ducs)
  
subject: use P instead of (x, y, z) in physics