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: execution_errors.rb
    path of file: ./repos/graphql-ruby/lib/graphql/execution/interpreter
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

module GraphQL
  module Execution
    class Interpreter
      class ExecutionErrors
        def initial
    
        def add(err_or_msg)
          err = case err_or_msg
          when String
            GraphQL::ExecutionError.new(err_or_msg)
          when GraphQL::ExecutionError
            err_or_msg
          else
            raise ArgumentError, "expected String or GraphQL::ExecutionError, not #{err_or_msg.class} (#{err_or_msg.inspect})"
          end