Return a generator that adds edges to the chart, one at a time. Each time the generator is resumed, it adds a single edge and yields that edge. If no more edges can be added, then it yields None.

If the parser’s strategy, grammar, or chart is changed, then the generator will continue adding edges using the new strategy, grammar, or chart.

Note that this generator never terminates, since the grammar or strategy might be changed to values that would add new edges. Instead, it yields None when no more edges can be added with the current strategy and grammar.