macro definitions.  (There's a portable implementation of
R5RS macros out there, but it's about twice the size of the entire
Ur-Scheme compiler.)  Similarly, I was stuck with a bunch of the
boneheaded design decisions of the 
Scheme built-in types &mdash; for example, no
auto-growing mutable containers, separate types for strings and
characters, and the difficulty of doing any string processing without
arithmetic and side effects.</p> </li>

<li><b>Start with the simplest thing that could possibly work.</b>