abstracts a certain set of properties from a huge number of
actual life cycles that shared those properties. When we talk
about life cycle models, it is often convenient to subdivide
them into stages (or phases), each with different concerns. A
commonly used set of stages is concept, development, production,
utilization, support and retirement, representing differing
goals and purposes at different points in the life cycle. It is
important to understand that different elements of a system may
transition from one stage to another at different points in
time. In fact, it is sometimes useful to consider stages that
are concurrent, such as utilization and support. Life cycle
models are useful because they can be applied in developing the
plan for a project; their usage guides the allocation of
resources appropriate to the different stages. When applied to a
project, stages are often viewed as being separated by decision
gates, a formal process for determining that the system, or an
element of the system, may now