Hi

I got the Error message when I copy and paste your solution or the solution from operator only 

restart;
`&||` := (R::seq(algebraic)) -> `if`(_npassed = 0, infinity, 1/`+`(`/`~(1, R)));
R1 &|| R2;
Error, mismatched or missing bracket/operator

Did I overlook anything? 

Tanks :) 

Thank you so much @Thomas Richard