Joyce Kilmer and most computer scientists agree: there is no poem as lovely as a tree. In our love affair with the tree it is parsed, pattern matched, pruned — and printed. A pretty printer is a tool, often a library of routines, that aids in converting a tree into text. The text should occupy a minimal number of lines while retaining indentation that reflects the underlying tree. A good pretty printer must strike a balance between ease of use, flexibility of format, and optimality of output.