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   One of the properties that can be associated to a field is the name of the
   file (target file) on which the decodification of information (N:1
   relation or Lookup) will take place. For more information regarding N:1
   relations consult the paragraph relative to this argument in this chapter.

   It is sufficient to specify the name of the target file and the index to
   use in order to be sure that each time a value is selected for a field,
   the other information related to it and contained in other files will also
   be available.

   For a lookup relation it is possible to:

     . define the lookup mode making the insertion of data in a
       field either mandatory or optional by choosing between: allows empty
       key, does not allow empty key and may be everything

     . define the name, the primary positioning key, the filter and
       break criteria for the search window to activate in order to consult
       the target file during a lookup.

   In order to define the above options, access the environment activated
   with the Options button. All values can be edited by pressing the
   Space key.
    The Alt-1 keys allow to build the expressions interactively.