Functions.Rmd: some words on identical()

Adds discussion of `identical()` function to `if` section. 

I, personally, find identical() to be useful, but I can see that it does not add aesthetic value to the code. 

(BTW, really enjoying seeing this book come together!)

functions.Rmd

@@ -279,6 +279,14 @@ if (NA) {}
 
 You can use `||` (or) and `&&` (and) to combine multiple logical expressions. These operators are "short-circuiting": as soon as `||` sees the first `TRUE` it returns `TRUE` without computing anything else. As soon as `&&` sees the first `FALSE` it returns `FALSE`. You should never use `|` or `&` in an `if` statement: these are vectorised operations that apply to multiple values. If you do have a logical vector, you can use `any()` or `all()` to collapse it to a single value.
 
+You can enforce this more-strictly by using the `identical()` function, which returns either a single `TRUE` or a single `FALSE`. One thing to watch out for is that *you* have to be more strict when specifying integers:
+
+```{r}
+if (i == 0){}
+
+if (identical(i, 0L)){}
+```
+
 ### Multiple conditions
 
 You can chain multiple if statements together: