(further) argument to `x`. ("K" for "constant".) Compare K
to our definition of `true`.
-> **S** is defined to be `\f g x. f x (g x)`. This is a more
+> **S** is defined to be `\f g x. f x (g x)`. This is a more
complicated operation, but is extremely versatile and useful
(see below): it copies its third argument and distributes it
over the first two arguments.