How do I mask out to the missing value flag?

[Lev Tarasoff <lev@xxxxxxxxxxxxxxxxxxxxxxxxxx>]

I'd appreciate some help with setting mask values to missing data. I
want to mask mask out the oceanic part of data sets. I've tried a
bunch of things including:

SET VARIABLE/BAD=-1000. $1
let $2 = IF hdk[d=hdKMS] GT (-20.) THEN $1 ELSE (-1000.)
!hdk is the topgraphic file

However, -1000 shows up in the averages when stats is issued. How do I
assign the missing value flag to $2 when the point is to be masked
out? (The ferret documentation seems to suggest that the missing value
flag is represented by &SHY but I get an error with this. What is the
symbolic representation for the missing value flag? Suggestion for
documentation: masking out data to missing should be given as an
explicit example, as I would think that this is a common enough need).

Thanks

Lev Tarasoff -  Dept of Physics, University of Toronto,
                60 St. George St., Toronto, Ontario, CANADA, M5S 1A7
                Tel (416)-946-3019  Fax (416)-978-8905
                email: lev@atmosp.physics.utoronto.ca