Add references for complex subassignment

Functions.Rmd

@@ -1277,7 +1277,11 @@ x <- `names<-`(`*tmp*`, `[<-`(names(`*tmp*`), 2, "two"))
 rm(`*tmp*`)
 ```
 
-(Yes, it really does create a local variable named `*tmp*`, which is removed afterwards.)
+(Yes, it really does create a local variable named `*tmp*`, which is removed afterwards.)[^subassignment]
+
+[^subassignment]: See [this StackOverflow answer](https://stackoverflow.com/a/10491881/946850) for an alternative explanation. At the time of writing, even [the R language definition](https://cran.r-project.org/doc/manuals/R-lang.html#Subset-assignment) isn't really specific about why it is necessary to use a temporary local variable, or how exactly complex assignment works with three or more levels.  See <https://rpubs.com/krlmlr/telescope> for an example that attempts to shed light on what happens under the hood.
+
+Note the use of `[<-`, which is the replacement variant of a special form introduced below.
 
 ### Special forms
 \index{special forms}
@@ -1292,6 +1296,13 @@ The subsetting operators:
 
 * `x[i]` (`` `[`(x, i) ``) 
 * `x[[i]]` (`` `[[`(x, i) ``)
+* `x$a` (`` `$`(x, "a") ``)
+
+The subassignment operators (a combination of the special and replacement forms):
+
+* `x[i] <- value` (`` x <- `[<-`(x, i, value) ``)
+* `x[[i]] <- value` (`` x <- `[[<-`(x, i, value) ``)
+* `x$a <- value` (`` x <- `$<-`(x, "a", value) ``)
 
 And the tools of control flow:
 

Subsetting.Rmd

@@ -499,6 +499,8 @@ x[c(1, 2)] <- c(101, 102)
 x
 ```
 
+This looks like `x` is updated in place, but a copy of `x` is created via the copy-on-modify rules (see Section \@ref(copy-on-modify)) if it is bound to more than one name. Section \@ref(replacement-functions) describes the details of complex assignments of the form `x[i1][i2] <- value`.
+
 I recommend that you should make sure that `length(value)` is the same as `length(x[i])`, and that `i` is unique. This is because, while R will recycle if needed, those rules are complex (particularly if `i` contains missing or duplicated values) and may cause problems.
 
 With lists, you can use `x[[i]] <- NULL` to remove a component. To add a literal `NULL`, use `x[i] <- list(NULL)`: