Hello! I came across with something that confused me while studying the book and I need some help. In section 7.3 , equation (7.3.4) should have the drivative of ε(x) in order to "vanish when ε(x) is constant" . But in the lines before this expression he says that the variation of action vanishes, not when ε(x) is constant but when the fields satisfy the dynamical equations. So why should (7.3.4) have the derivative of ε(x), since its constant value is not a criterion for vanishing variation? Also, some lines below, he deals with the case that ε(x) is not constant, but in equations (7.3.9) and (7.3.13) he treats ε(x) like a constant! What am I missing here???