Not a typo! Think of f(x) = x as a seed function that can be used to build other functions. It's one way to avoid talking about "variables" as a "data type" and just keep everything about functions. We can make a function like x + x*exp(log(x)) by "formally" writing

    f + f*(exp∘log)

where + and * are understood to produce new functions. Sort of Haskell-y.