I had some attributes whose values were very out of range and because I was in a hurry I couldn't go back to the source data and eliminate them there. What I wanted to do was to set the value to missing if it was greater than a certain value.
So I used the Generate Attributes operator like so.
In other words if (a1 > 5) then set a1 to missing (by dividing 0 by 0).
It's OK to modify the value of an existing attribute and not create a new attribute at all. I'm sure this used not to work so an enhancement has sneaked in - maybe my memory is playing tricks - I don't mind though - it works.