# File lib/rmail/header.rb, line 591
    def param(field_name, param_name, default = nil)
      if field?(field_name)
        params = params_quoted(field_name)
        value = params[param_name]
        return Utils.unquote(value) if value
      end
      if block_given?
        yield field_name, param_name
      else
        default
      end
    end