Listing 6: The implementation of operator+

template  <class ExprT1,class ExprT2>
BinaryExpr<ExprT1,ExprT2,plus<double> >
operator+(ExprT1 e1, ExprT2 e2) 
{ return BinaryExpr<ExprT1,ExprT2,plus<double> >(e1,e2); } 
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