This feature is not supported in SDL-2000.

In 'Basic system structure' it was stated that a block contains processes, interconnected by signal routes. Sometimes further structuring of a block is necessary, so that a block may have a substructure. An example is given in figure 28. The block B contains the process types P1 and P2. In addition, it contains a substructure, referenced by the little rectangle containing the name B of the block substructure.

Figure 28: Block diagram with substructure reference

The substructure is described by a substructure diagram in figure 29. The substructure diagram is similar to a system diagram (except for the channels connected to the block). In figure 29 it contains the blocks B1, B2 and B3.

Figure 29: Block substructure diagram

The interface of the substructure to its environment must be the same as that of the block.

Figure 30: Shorthand for block partitioning

A block definition containing process definitions corresponds to a leaf-block in the system. A block definition containing a substructure definition corresponds to a partitioned block in the system. A choice between these two versions of a block must be made before interpretation time. The result is called consistent partitioning subset. There are some rules for the selection of a consistent partitioning subset, ensuring that it corresponds to a valid block tree (e.g. each branch must terminate in a leaf-block, each block must be connected, directly or indirectly, to the root).

If a block contains only a substructure and no processes, there is a shorthand as shown in figure 30. In this example, block B1 does not contain any processes, but is partitioned into the blocks B11, B12 and B13.

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