Prevent cloning of clones

[Adde]

I'm trying to create a representations for different units of 
measurement in SVG and the best i could come up with was the following:

SVG addPrototype: #Unit derivedFrom: {Cloneable}.
SVG Unit addSlot: #postfix.
SVG Unit traits addSlot: #Pixels valued: (SVG Unit clone `>> [postfix: 
'px'.]).
u@(SVG Unit traits) toXML [ u postfix ].

However, there is no point in doing 'SVG Unit Pixels clone' so I was 
wondering how I can make it behave like numbers, ie. '1 clone' gives an 
error message.

/Adde