(do ..forms)
  forms: zero or more forms

Performs sequential evaluation.

When the do form is evaluated, each of its forms are evaluated in order from start to finish. The final form's result is returned.

Many other forms establish an "implicit do". For example, the body of a function:

(defn example (a)

With no arguments, (do) evaluates to #n. An empty "implicit do" also evaluates to #n:

(let empty-fn (fn ()))
(prn (empty-fn)) ; prints #n