(see below): it copies its third argument and distributes it
over the first two arguments.
-> **get-first** was our function for extracting the first element of an ordered pair: `\fst snd. fst`. Compare this to `K` and `true` as well.
+> **fst** was our function for extracting the first element of an ordered pair: `\a b. a`. Compare this to `K` and `true` as well.
-> **get-second** was our function for extracting the second element of an ordered pair: `\fst snd. snd`. Compare this to our definition of `false`.
+> **snd** was our function for extracting the second element of an ordered pair: `\a b. b`. Compare this to our definition of `false`.
> **B** is defined to be: `\f g x. f (g x)`. (So `B f g` is the composition `\x. f (g x)` of `f` and `g`.)