code: def apply_mutation(module_path, operator, occurrence):
      
      module_ast = get_ast(module_path, python_version=operator.python_version)
      original_code = module_ast.get_code()
      visitor = MutationVisitor(occurrence, operator)
      mutated_ast = visitor.walk(module_ast)
  
      mutated_code = None
      if visitor.mutation_applied:
          mutated_code = mutated_ast.get_code()
          with module_path.open(mode='wt', encoding='utf-8') as handle:
              handle.write(mutated_code)
              handle.flush()
  
      return original_code, mutated_code
docs: Apply a specific mutation to a file on disk.
  
      Args:
          module_path: The path to the module to mutate.
          operator: The `operator` instance to use.
          occurrence: The occurrence of the operator to apply.
  
      Returns: A `(unmutated-code, mutated-code)` tuple to the with-block. If there was
          no mutation performed, the `mutated-code` is `None`.