Thank you. "modify an object" means "change the number of elements", since you obviously want to be able to manipulate the individual elements of a container a

Thank you. "modify an object" means "change the number of elements", since you obviously want to be able to manipulate the individual elements of a container as you iterate over them. The object here is the container, not the elements themselves. I can't comment on other languages, but I'd say that guideline is a little too strict for Go. The classic implementation of Breadth First Search involves iterating over a queue as you fill it. "don't modify the RHS while in a range clause" would be a more suitable guideline for Go. Note that it's subtley different from "iterating over" - indeed, the answer to my bug was to iterate without using the range clause: for i := 0; i < len(input); i++{ if newValue := test(input); newValue != nil { input = append(input, newValue) } } I now appreciate the difference between this and the range clause - the length is evaluated every iteration this way. The range clause evaluates it once, at the beginning - rule (1).