limitations to missing values ?

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Hi,
To my surprise a MISSING VALUES does not work.

I ave a metric variable with the missing values 7 8 9 and want to add 
the value of 3 to the list of missings.

So, I tried first the first command, and as this did not work  then the 
others in this list :
miss val political (3, 7 , 8, 9).
miss val political (3 thru 9).
miss val political (3 thru hi).

And never the 3 becomes missing !

Are there limits to the number of missing a variable can have ? I looked 
into the doc , but did not find info about an eventual limit.

Any idea appreciated.

- ftr