The process structure is represented as a tree similar in form to a standard organisation diagram or program structure chart. The root node (at the top of the tree) represents the process as a whole, and is initially shown alone, with no further detail. Any leaf node for which more information is available can be expanded by double-clicking with Mouse-1; double-clicking a node which is currently expanded will cause that part of the tree to be "folded up".
When a leaf node is expanded, branches are added according to the number of sub-components of the node in question. Thus, a node labelled with a parallel composition symbol `[|..|]' will expand to have two children representing the sub-processes which are combined in parallel. Each child will be associated with its own contribution to the overall erroneous behaviour being examined.(6)
If any of the compression or factorisation operators (see 5.1 Using Compressions) were applied in building up a system, the process of extracting the back-trace information may involve further refinement checks and thus significant computation. To indicate this, nodes of the process structure corresponding to compressed processes will be coloured red, and will not be expanded by default. Examining their internal behaviour is still straightforward, however: simply double click on the coloured node.
(Note: some lesser-used CSP constructions, such has the repetition operator * of [Hoare85], have a single syntactic sub-component which may be responsible for more than one independent sub-behaviour. In this case, the process tree will contain a branch for each independent behaviour. There are currently no operators which involve both multiple processes and multiple behaviours!)
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