In most other programming languages, new abstractions are built by writing code on /top/ of existing abstractions. But in lisp, its possible to build new abstractions by writing code beneath existing code. Abstract: A partial evaluator automatically specializes a program with respect to some of its input. This article shows how the idea comes up naturally when you write program generators by hand, then builds a basic online partial evaluation library and puts it to work transforming an interpreter into a compiler. 0. Introduction Mainstream programmers think of writing an interpreter or a compiler as a major job, worth doing only for a major problem. They know this because the languages they use every day have big, serious implementations, and the compiler class they took in school had them write just one big, semi-serious compiler. Lispers know better: all the textbooks show how to write a Lisp interpreter in about a page of code.