You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: node.rb
    path of file: ./repos/mutant/lib/mutant/mutator
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

module Mutant

  # Generator for mutations
  class Mutator

    # Abstract base class for node mutators
    clas
    
      def mutate_child(index, &block)
        block ||= TAUTOLOGY
        mutate(node: children.fetch(index), parent: self).each do |mutation|
          next unless block.call(mutation)
          emit_child_update(index, mutation)
        end
      end