don't split hidden blocks from clamp of backward block
don't split hidden blocks from clamp of backward block

tradzik/musicblocks,Kachachan/musicblocks,hemantkasat/musicblocks,tradzik/musicblocks,hemantkasat/musicblocks,Kachachan/musicblocks,tradzik/musicblocks,Kachachan/musicblocks,hemantkasat/musicblocks