To add from the left, we have to look ahead to see whether the next addition will require an exchange.
In Subtracting From The Left, We Look Ahead To See Whether A I In Any Denomination Must Be Reserved For Changing; Thus In Subtracting 274 From 637 We Should Put Down 2 From 6 As 3, Not As 4, And 7 From 3 As 6.
When using look-ahead assertions and negations, this can all get even trickier.
We need to look ahead at building social cohesion.
With look ahead map based support some graceful degradation could be provided.