EXTENT_ALIGN
newstart = EXTENT_ALIGN(substart, alignment, skew);
newstart = EXTENT_ALIGN((last->er_end + 1), alignment, skew);
dontcross = EXTENT_ALIGN(newstart+1, boundary,
newstart = EXTENT_ALIGN((rp->er_end + 1), alignment, skew);
dontcross = EXTENT_ALIGN(newstart+1, boundary,