def create_lambda(arg)
      case arg
      when Proc   then arg
      when Symbol then lambda { |request| request[arg] }
      else fail "Canot create a lambda expression from this argument: #{arg.inspect}!"
      end
    end
def create_lambda(arg)
      case arg
      when Proc   then arg
      when Symbol then lambda { |request| request[arg] }
      else fail "Canot create a lambda expression from this argument: #{arg.inspect}!"
      end
    end
Creates a lambda expression to return a static field from a request. If the
 argument already is a lambda exprssion, it will simply return the argument.