Dbf:AddRelation(xKey,cAlias,nOrder): public: return true
  Last created object OneDbf is appended with next relation definition.

  Parameter description:
  ~~~~~~~~~~~~~~~~~~~~~~
  cKey: character: no default
   Key expresion as text string, for making relation is this evaluated
   for SEEK command.

  cAlias: character: no default
   Work area selection for SEEK command.

  nOrder: numeric: no default
   Order of active index in "cAlias" workarea.