I have a habit of thinking up wierd instrument ideas in my spare time. One of these is to place fretted and fretless strings on the same fingerboard. Or have a portion of a single string marked with frets, and the rest fretless. Are there any technical difficulties with placing a fret accurately over only a certain part of the fingerboard, considering everthing should be nice and pretty? Any other issues? Would having fretlines over the unfretted strings simplify the luthier's job, seeing as they could then cut the entire slot? Most importantly for my ego, has anyone done this already?