mkExprs xs = [ev | (ys,zs) <- unmerges xs
                  ,ev1 <- mkExprs ys
                  ,ev2 <- mkExprs zs
                  ,ev  <- combine ev1 ev2]
mkExprs xs = [ev | (ys,zs) <- unmerges xs
                  ,ev1 <- mkExprs ys
                  ,ev2 <- mkExprs zs
                  ,ev  <- combine ev1 ev2]
mkExprs xs = [ev | (ys,zs) <- unmerges xs
                  ,ev1 <- mkExprs ys
                  ,ev2 <- mkExprs zs
                  ,ev  <- combine ev1 ev2]