The introducing sentence is:
As an example of the heuristics that are implemented in the algorithm to update a context variable,
The example is:
the main division surrounding an element is used as the value of the curblock variable.
I don't know how can I relate them to each other through one or two sentences.
Update: I think of this solution:
The way we update the current block is an example of the heuristics that are implemented in the algorithm. To do so, we use the main division surrounding an element as the value of the current block.