I think the author is talking about minor variations on Lisp syntax that include indentation-sensitivity, not about different languages that are homoiconic. Such variant syntaxes include i-expressions (<a href="http:&#x2F;&#x2F;srfi.schemers.org&#x2F;srfi-49&#x2F;srfi-49.html" rel="nofollow">http:&#x2F;&#x2F;srfi.schemers.org&#x2F;srfi-49&#x2F;srfi-49.html</a>) and sweet-expressions (<a href="http:&#x2F;&#x2F;readable.sourceforge.net&#x2F;" rel="nofollow">http:&#x2F;&#x2F;readable.sourceforge.net&#x2F;</a>), which the author mentions in the Acknowledgements. For example, here are some sweet-expressions:<p><pre><code>  define factorial(n)
    if {n &lt;= 1}
      1
      * n factorial(- n 1)
</code></pre>
This desugars to:<p><pre><code>  (define (factorial n)
    (if (&lt;= n 1)
      1
      (* n (factorial (- n 1)))))
</code></pre>
I think the Lisp power that the author is referring to is homoiconicity, and the corresponding ease of writing macros. For example, i-expressions sacrifice homoiconicity in some contexts.<p>But the author seems to have overlooked that sweet-expressions, which were developed later, do not sacrifice homoiconicity. A human reader can still easily understand the AST from reading sweet-expression syntax. The real reasons that sweet-expressions have faltered relate more to the increased difficulty of collaborating with other developers, the lack of editor plugins that highlight sweet-expressions correctly, and incompatibility with Clojure syntax.