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 CHANGED:  TBrowse error handling
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     Assigning invalid data to TBrowse and TBColumn instance variables would
     not generate an error in version 5.01. Type checking during instance
     variable assignment has been added in CA-Clipper 5.2 to trap invalid
     values and produce a recoverable runtime error.

     The problem commonly occurs when assigning a NIL value to an instance
     variable as a result of an optional parameter:

     FUNCTION MyBrowse( nTop, nLeft, nBottom, nRight, cHeadSep )

        oBrowse := TBrowseNew( nTop, nLeft, nBottom, nRight )
        // If only four arguments are passed to this function the following
        // line will assign a NIL to headSep, resulting in an error.
        oBrowse:headSep := cHeadSep

     The following example prevents accidentally assigning a NIL to a TBrowse
     instance variable when a function accepts optional parameters:

     FUNCTION MyBrowse( nTop, nLeft, nBottom, nRight, cHeadSep )

        oBrowse := TBrowseNew( nTop, nLeft, nBottom, nRight )
        // don't assign a NIL - headSep has a default value.
        IF cHeadSep != NIL
           oBrowse:headSep := cHeadSep
        ENDIF