Problem with value? and system/words/var in Rebol

Question

Why does clipboard and system/words/clipboard points to 2 different value ?

>> clipboard
** Script Error: clipboard has no value
** Near: clipboard
>> system/words/clipboard
>> value? 'clipboard
== false
>> value? 'system/words/clipboard
== true
>>

Then how can I use value? within a func which uses clipboard as refinement if I can't test it on system/words/clipboard ?

Answer 1

value? in system/words 'clipboard

Answer 2

Found a solution

unset? system/words/clipboard

but this is hack, Gregory's solution is more elegant.