polycalc | PolyCalc : Polynomial Calculator | Math library

As indicated in the answer to the question you linked, it is hard to specify the operator precedence of juxtaposition because there is no operator to shift. (As in your code, you can specify the precedence of the production expr: expr expr. But what lookahead token will this reduction be compared with? Adding every token in FIRST(expr) to your precedence declarations is not very scalable, and might lead to unwanted precedence resolutions.

An additional problem with the precedence solution is the behaviour of the unary minus operator (an issue not addressed in the linked question), because as written your grammar allows a - b to be parsed either as a subtraction or as the juxtaposed multiplication of a and -b. (And note that - is in FIRST(expr), leading to one of the possibly unwanted resolutions I referred to above.)

So the best solutions, as recommended in the linked question, is to use a grammar with explicit precedence, such as the following: (Here, I used juxt as the name of the non-terminal, rather than expr_sequence):