1. A method for merging a plurality of sub-resolution assist features, comprising: receiving a mask pattern for a photolithographic mask, the mask pattern comprising a plurality of sub-resolution assist features of one or more priorities; and repeating the following using a computer for each priority of the one or more priorities: identifying a current set of current sub-resolution assist features to be merged, the current set comprising sub-resolution assist features of a current priority; merging at least two current sub-resolution assist features according to a merge bar width and a current distance between the at least two current sub-resolution assist features; updating the current set to yield a next set of next sub-resolution assist features to be merged, the next set comprising at least one sub-resolution assist feature of the current priority and at least one sub-resolution assist feature of a previous priority; merging at least two next sub-resolution assist features according to the merge bar width and a next distance between the at least two next sub-resolution assist features; and setting a next priority as the current priority.