A processing system is described which induces a context free grammar (CFG) based on a set of descriptions. The descriptions pertain to a particular subject. Thus, the CFG targets the particular subject, and is accordingly referred to as a subject-targeted context free grammar (ST-CFG). The processing system can use the ST-CFG to determine whether a new description is a proper description of the subject. The processing system also provides synthesizing functionality for building an ST-CFG based on one or more smaller component ST-CFGs.