> concatenation is associative while subtraction is left-associative Yes, exactly my point: `sum ::= term '-' sum` is right-associative, and therefore not a co

> concatenation is associative while subtraction is left-associative Yes, exactly my point: `sum ::= term '-' sum` is right-associative, and therefore not a correct syntax for subtraction. Similarly, place-value notation[0] is left-associative, so `nat ::= digit | digit nat`, which is right-associative, is not a valid grammer for a place-value-notation natural number. 0: well, big-endian place-value notation, which is what we're talking about here