Remarks

    merge can perform both one-to-one and match merges.  In either case, the variable _merge (or the variable specified in
    _merge() if provided) is added to the data containing

                               _merge==1    obs. from master data                           
                               _merge==2    obs. from only one using dataset                 
                               _merge==3    obs. from at least two datasets, master or using

    update can be used only when there is one using file.  When update is specified, the codes for _merge are

                               _merge==1    obs. from master data                           
                               _merge==2    obs. from using data                             
                               _merge==3    obs. from both, master agrees with using         
                               _merge==4    obs. from both, missing in master updated        
                               _merge==5    obs. from both, master disagrees with using      

    When multiple using files are specified, a set of summary variables is created, as long as nosummary is not used.
    These summary variables are named _merge1 (related to the first using dataset), _merge2 (related to the second using
    dataset), etc. (or, once again, the variable specified in _merge() if provided, followed by the number of the using
    file).  These variables will contain

                               _mergek==0   obs. not present in corresponding using dataset  
                               _mergek==1   obs. present in corresponding using dataset      

    Variable labels identifying the dataset associated with each summary variable are attached to these summary variables.