Step 6: Determine the state of the exposed submatrices

The exposed submatrices are what will be updated. To determine how to update them requires us to first determine what their contents are immediately after the repartitioning that exposed them.

The key here is to realize that the double lines in the repartitioning have semantic meaning:

,

Thus, by substituting the exposed matrices into the appropriate parts of the loop-invariant, we derive a predicate that indicates the contents of the different submatrices:


becomes
.

Finally, by simplifying the expression on the right of the equal sign, we determine the state of the submatrices of B :

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