sequential yes/no

[markvanderloo]

Should guarding expressions be evaluated inside or outside the loop? Make an option to decide that.

[edwindj]

For the sequential evaluation, the current implementation can be optimized:

Current implementation does a lot of copying: each modifier makes a selection in a data.frame (copy), edits the selection (copy) and writes it back.

For the sequential evaluation, putting all guards and modifiers in the same within would result in only one copy of the each modified column not matter how many modifiers
But this could very well be premature optimization / non problem , not sure how the performance currently is.