def qualify(*flags)
      flags.each { |flag| set_flag(flag) }
      # propagate to restrictions
      if @restrictions then @restrictions.each { |prop| prop.qualify(*flags) } end
    end
def qualify(*flags)
      flags.each { |flag| set_flag(flag) }
      # propagate to restrictions
      if @restrictions then @restrictions.each { |prop| prop.qualify(*flags) } end
    end
Qualifies this attribute with the given flags. Supported flags are listed in {SUPPORTED_FLAGS}.

 @param [<Symbol>] the flags to add
 @raise [ArgumentError] if the flag is not supported