Program understanding efforts by individual maintainers are dominated by a process known as discovery, which is characterized by low-level searches through the source code and documentation to obtain information that is important to the maintenance task. Discovery is complicated by the delocalization of information in the source code, and can consume from 40-60% of a maintainer's time. This paper presents an ontology for representing code-level knowledge based on abstract syntax trees, that was developed in the context of studying maintenance problems in a small software company. The ontology enables the utilization of automated reasoning to counter delocalization, and thus to speed up discovery.