dissolveOvr

function dissolveOvr( clip main, clip ovr, int "frameStart", int "frameIn", int "frameOut", int "frameEnd" )
{
    frameStart = default(frameStart,0)
    frameIn = max(default(frameIn,5),frameStart)
    frameEnd = default(frameEnd,ovr.frameCount-1)
    frameOut = min(default(frameOut,ovr.frameCount-5),frameEnd)

    start = frameStart > 0
    in = frameStart < frameIn
    mid = frameIn < frameOut
    out = frameOut < frameEnd
    end = frameEnd < main.framecount-1

    vStart = start ? main.trim(0,frameStart-1) : ""
    vIn = in ? dissolve(main.trim(frameStart,frameIn-1),ovr.trim(frameStart,frameIn-1),frameIn-frameStart) : ""
    vMid = mid ? ovr.trim(frameIn,frameOut) : ""
    vOut = out ? dissolve(ovr.trim(frameOut+1,frameEnd),main.trim(frameOut+1,frameEnd),frameEnd-frameOut) : ""
    vEnd = end ? main.trim(frameEnd+1,0) : ""

    start ? vStart : nop
    in ? (isclip(last) ? last+vIn : vIn) : nop
    mid ? (isclip(last) ? last+vMid : vMid) : nop
    out ? (isclip(last) ? last+vOut : vOut) : nop
    end ? (isclip(last) ? last+vEnd : vEnd) : nop
}