Many statistical operations in Survo assume that the user has
selected the variables and indicated their tasks in the analysis by
activating the fields in the data file (for example, by key FILE_ACT)
or by writing either a VARS or a MASK specification in the current
edit field.
If both VARS and MASK is given, the selection provided by VARS is
used.
1 = VARS specification
2 = MASK operation and specification
M = Setting masks by FILE MASK
O = Selecting observations
C = More information on control operations
S = More information on statistical operations
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