nhưng nó có thể có nhiều trẻ em, vì vậy childNodes thực sự là một danh sách các nút có chứa tất cả các trẻ em của phần tử, để nguồn. Một khi chúng ta có một tham chiếu đến các yếu tố neo, chúng ta có thể nhận được một tham chiếu đến đoạn cha mẹ của nó bằng cách sử dụng parentNode | 66 Chapter 4 Software processes Figure rhe software life cycle is then transformed using mathematical transformations that preserve its consistency into executable code. The best-known example of a formal development process is the Cleanroom process which was originally developed by IBM Mills et al. 1987 Selby et al. 1987 Linger 1994 Prowell et al. 1999 . In the Cleanroom process each software increment is formally specified and this specification is transformed into an implementation. Software correctness is demonstrated using a formal approach. There is no testing for defects in the process and the system testing is focused on assessing the system s reliability. Both the Cleanroom approach and another approach to formal development based on the B method Wordsworth 1996 are particularly suited to the development of systems that have stringent safety reliability or security requứements. The formal approach simplifies the production of a safety or security case that demonstrates to customers or certification bodies that the system does actually meet the safety or security requirements. Outside of these specialised domains processes based on formal transformations are not widely used. They require specialised expertise and in reality for the majority of systems this process does not offer significant cost or quality advantages over other approaches to system development. The waterfall model_ The first published model of the software development process was derived from more general system engineering processes Royce 1970 . This is illustrated in Figure . Because of the cascade from one phase to another this model is known as the waterfall model or software life cycle. The principal stages of the model map onto fundamental development activities Software process models 67 1. Requirements analysis and definition The system s services constraints and goals are established by consultation with system users. They
