The insertion operator $(<<).$ The insertion operator should add the int parameter into the receiving IntCollection. The functionality is the same as the add$()$ function, i$.$e$.$ add ints to the collection. Note, however, that this function must return a reference to itself in order to support multiple insertions on the same line, e$.$g$.$ c $<<45<<-210.$ Unlike the assignment operator, this return must be done by reference, because each insertion actually modifies the IntCollection object, and insertion is done from left to right.