exprs ns = [e | (ls,rs) <- split ns,
                l       <- exprs ls,
                r       <- exprs rs,
                e       <- combine l r]
exprs ns = [e | (ls,rs) <- split ns,
                l       <- exprs ls,
                r       <- exprs rs,
                e       <- combine l r]
exprs ns = [e | (ls,rs) <- split ns,
                l       <- exprs ls,
                r       <- exprs rs,
                e       <- combine l r]