unction <code>f</code> on argument <code>e</code>.</p>
<p><em>Remark</em>: the non-standard use of square brackets is to distinguish at the syntax level the applications for the refence terms below… In the concrete syntax the parentheses may be used, the Lisp way as: <code>(f e)</code>. Since we know that all abstrations have a single argument, the following syntactic shortcuts are quite useful:</p>
<pre><code class="language-clojure">(e1 e2) ≡ [e1 e2]  ;; if e1 is not a defined term
(e1 e2 e3) ≡ [[e1