[Prolog] operator x->y is hastily introduced and then used in a complex example

`->` is introduced in `countUpTo2` as a "more elegant syntax" equivalent to `countUpTo`, but its meaning is not really explained, and the next example `safe` flies over the reader's head because of that.

Locate the Prolog document containing the countUpTo, countUpTo2, and safe examples, then read those examples together. Done means the meaning of -> is explained before safe and the example no longer assumes readers understand the operator from context.

Written by the indexing model from the issue text.