    # File lib/rmail/header.rb, line 617
        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