old_contents: #!/usr/bin/env python3
  """Classes and methods for working with PDA stacks."""
  
  
  class PDAStack(object):
      """A PDA stack."""
  
      def __init__(self, stack, **kwargs):
          """Initialize the new PDA stack."""
          if isinstance(stack, PDAStack):
              self._init_from_stack_obj(stack)
          else:
              self.stack = list(stack)
  
      def _init_from_stack_obj(self, stack_obj):
          """Initialize this Stack as a deep copy of the given Stack."""
          self.__init__(stack_obj.stack)
  
      def top(self):
          """Return the symbol at the top of the stack."""
          if self.stack:
              return self.stack[-1]
          else:
              return ''
  
      def pop(self):
          """Pop the stack top from the stack."""
          self.stack.pop()
  
      def replace(self, symbols):
          """
          Replace the top of the stack with the given symbols.
  
          The first symbol in the given sequence becomes the new stack top.
          """
          self.stack.pop()
          self.stack.extend(reversed(symbols))
  
      def copy(self):
          """Return a deep copy of the stack."""
          return self.__class__(self)
  
      def __len__(self):
          """Return the number of symbols on the stack."""
          return len(self.stack)
  
      def __iter__(self):
          """Return an interator for the stack."""
          return iter(self.stack)
  
      def __repr__(self):
          """Return a string representation of the stack."""
          return '{}({})'.format(self.__class__.__name__, self.stack)
  
      def __eq__(self, other):
          """Check if two stacks are equal."""
          return self.__dict__ == other.__dict__
  
new_contents: #!/usr/bin/env python3
  """Classes and methods for working with PDA stacks."""
  
  
  class PDAStack(object):
      """A PDA stack."""
  
      def __init__(self, stack):
          """Initialize the new PDA stack."""
          self.stack = list(stack)
  
      def top(self):
          """Return the symbol at the top of the stack."""
          if self.stack:
              return self.stack[-1]
          else:
              return ''
  
      def pop(self):
          """Pop the stack top from the stack."""
          self.stack.pop()
  
      def replace(self, symbols):
          """
          Replace the top of the stack with the given symbols.
  
          The first symbol in the given sequence becomes the new stack top.
          """
          self.stack.pop()
          self.stack.extend(reversed(symbols))
  
      def copy(self):
          """Return a deep copy of the stack."""
          return self.__class__(**self.__dict__)
  
      def __len__(self):
          """Return the number of symbols on the stack."""
          return len(self.stack)
  
      def __iter__(self):
          """Return an interator for the stack."""
          return iter(self.stack)
  
      def __repr__(self):
          """Return a string representation of the stack."""
          return '{}({})'.format(self.__class__.__name__, self.stack)
  
      def __eq__(self, other):
          """Check if two stacks are equal."""
          return self.__dict__ == other.__dict__
  
subject: Remove copy constructor for PDAStack