question: private Object executePlus ( PageContext pc , SQL sql , Query qr , Operation2 expression , int row ) throws PageException { Object left = executeExp ( pc , sql , qr , expression . getLeft ( ) , row ) ; Object right = executeExp ( pc , sql , qr , expression . getRight ( ) , row ) ; try { return new Double ( Caster . toDoubleValue ( left ) + Caster . toDoubleValue ( right ) ) ; } catch ( PageException e ) { return Caster . toString ( left ) + Caster . toString ( right ) ; } }
target:  execute a plus operation