code: func (m ModuleInstance) Child(name string, key InstanceKey) ModuleInstance {
  	ret := make(ModuleInstance, 0, len(m)+1)
  	ret = append(ret, m...)
  	return append(ret, ModuleInstanceStep{
  		Name:        name,
  		InstanceKey: key,
  	})
  }
docs: // Child returns the address of a child module instance of the receiver,
  // identified by the given name and key.