Many processes can be seen as transformations of tree-like data structures. In compiler construction, for example, we continuously manipulate trees and perform tree transformations: parse trees, abstract syntax trees, tree transformations, etc. This paper introduces a pattern matching compiler (TOM): a set of primitives which add pattern matching facilities to imperative languages such as C, Java, or Eiffel. We show that this tool is extremely non-intrusive, lightweight and useful to implement tree transformations. It is also flexible enough to allow the reuse of existing data structures.