def []=(flag, value)
      if o = option(flag)
        o.value = value
      else
        raise ArgumentError, "no option with flag `#{flag}'"
      end
    end
def []=(flag, value)
      if o = option(flag)
        o.value = value
      else
        raise ArgumentError, "no option with flag `#{flag}'"
      end
    end
Set the value for an option. Raises an ArgumentError if the option
 does not exist.