One way to do it would be to have a default associativity between different operators, but chains of the same operator can override it, e.g. suppose “⧺” means c

One way to do it would be to have a default associativity between different operators, but chains of the same operator can override it, e.g. suppose “⧺” means concatenation (rightward), “×” means repetition (leftward), and the default is leftward: [1, 2] ⧺ [3, 4] ⧺ [5, 6] × 2 × 3 (([1, 2] ⧺ ([3, 4] ⧺ [5, 6])) × 2) × 3 [ [ [1…6], [1…6] ], [ [1…6], [1…6] ], [ [1…6], [1…6] ] ] I dunno how good an idea that would be in practice, though.