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WHEN EXCEPTION IN ～ USE ～ END WHEN ☆

    WHEN EXCEPTION IN
    　　when-body
    USE
    　　exception-handler
    END WHEN

When-body
The portion between a WHEN-EXCEPTION-IN line and a USE line is called a when-body.
If the when-body has been completed normally, the cotrol proceeds to the the next line of the END-WHEN. Otherwise, the control branches to the exception-handler.
Exception-handler
The lines written between a USE line and a END-WHEN line is called an exception-handler.
If the control has reached the END-WHEN line, the control escapes the when-block and proceeds to the next line.
No branch out of an exception-handler using GOTO or IF-THEN-line_number is allowed.

Restrictions on BASICAcc
The following statements may not work, especailly for exceptions caused in inferior exception-handlers.
`RETRY`
resumes the statement that has caused the exception.
`CONTINUE`
resumes the next, provided that the line where the exception has occurred is the beginning or part of a block, the control proceeds to the next line of the block. For example, if an exception has occurred in a FOR line, the control proceeds to the next line of the NEXT line. If an exception has occurred in a ELSEIF-line, the control proceeds to the next line of the corresponding END-IF.