Now, `nil.to_s` returns a fixed instance: ```ruby nil.to_s.object_id # => 440 nil.to_s.object_id # => 440 nil.to_s.object_id # => 440 ... ``` This is useful when we have some variable `foo` which may be either `nil` or a string, and we want to check its emptiness in a condition: ```ruby if foo.to_s.empty?; ... end ``` By this feature, we do not (need to) create a new instance of an empty string each time we check `foo`, even when it happens to be `nil`. There are similar situations with arrays and hashes. We may have variable `bar` which may be either `nil` or an array, or `baz` which may be either `nil` or a hash, and we want to check their emptiness in conditions as follows: ```ruby if bar.to_a.empty?; ... end ``` ```ruby if baz.to_h.empty?; ... end ``` But unlike `nil.to_s`, the methods `nil.to_a` and `nil.to_h` create new instances of empty array or hash each time they are called: ```ruby ``` nil.to_a.object_id # => 540 nil.to_a.object_id # => 560 nil.to_a.object_id # => 580 ... nil.to_h.object_id # => 460 nil.to_h.object_id # => 480 nil.to_h.object_id # => 500 ... ``` The fact that this is somewhat inefficient discourages the use of `foo.to_a` or `foo.to_h` in such use cases. I request `nil.to_a` to `nil.to_h` to return a fixed empty instance.