The string INTEGER is returned if the symbol is equated to an
    integer, or if the symbol is equated to a string whose characters
    form a valid integer.

    If the symbol has been produced by a call to the F$CONTEXT
    function with a context type of PROCESS or by a call to the
    F$PID function, the string returned is PROCESS_CONTEXT. A symbol
    retains this type until F$CONTEXT is called with the symbol and
    the CANCEL keyword, or until a null string ("")  is returned by a
    call to F$PID.

    Similarly, the return value is the string CLUSTER_SYSTEM_CONTEXT
    for symbols created by the F$CSID function.

    If the symbol is a context symbol, then the return value will be
    one of the types shown in the following table.

    Symbol Type          Lexical Creating Symbol

    PROCESS_CONTEXT      F$PID or F$CONTEXT (with PROCESS context
                         type)

    CLUSTER_SYSTEM_      F$CSID
    CONTEXT

    The string STRING is returned if the symbol is equated to a
    character string whose characters do not form a valid integer
    or whose type is not a context.

    If the symbol is undefined, a null string is returned.