Does the CXlOper contain a string?
[C++]
BOOL IsString() const;
Returns TRUE if the CXlOper contains a string value, FALSE otherwise.
Header: xllplus.h
CXlOper Class 
					  | CXlOper Methods			 
					  | CXlOper::IsArray()			 
					  | CXlOper::IsDouble()			 
					  | CXlOper::IsError()			 
					  | CXlOper::IsBool()			 
					  | CXlOper::IsEmpty()			 
					  | CXlOper::IsMissing()			 
					  | CXlOper::IsInt()			 
					  | CXlOper::IsSRef()			 
					  | CXlOper::IsMRef()			 
					  | CXlOper::IsRef()