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?
    Existence test:  The "?" unary operator may be used to test for the
    existence of a symbol.


         ?id

    The result of this expression is 1 if "id" is a symbol known to the
    debugger and 0 otherwise.  If the symbol does not exist in the current
    scope then it must be qualified with its module name.  Automatic symbols
    exist only in the current subprogram.