A [command] that is part of [BLT]. Has the following subcommands:
    :   '''[drag&drop source]'''
    :   '''drag&drop source''' ''window'' ?''options''?
    :   '''drag&drop source''' ''window'' '''handler''' ?''dataType''? ?''command arg arg...''?
    :   <<br>>
    :   '''[drag&drop target]'''
    :   '''drag&drop target''' ''window'' '''handler''' ?''dataType command arg arg...''?
    :   '''drag&drop target''' ''window'' '''handle''' ''dataType'' ?''value''?
    :   <<br>>
    :   '''[drag&drop token]''' ''window''
    :   <<br>>
    :   '''[drag&drop drag]''' ''window x y''
    :   '''[drag&drop drop]''' ''window x y''
    :   '''[drag&drop active]'''
    :   '''[drag&drop errors]''' ?''proc''?
    :   '''[drag&drop location]''' ?''x y''?


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