Complex sentence: F however is higher-order function because the argument "f" is itself called as a function, so the programming language must support passing functions as arguments and also support function literals because FIX F is an expression rather than an identifier.
Split into simpler sentences: F however is higher-order function because the argument "f" is itself called as a function, so the programming language must allow passing functions as arguments. It must also allow function literals because FIX F is an expression rather than an identifier.