In response to programmer input of a trigger token, a program editor creates a pair of boundary tokens encapsulating an embedded lexical context and positions an insertion point within the embedded lexical context. The operations on the embedded lexical context are preferably performed using an embeddable sub-editor specialized to the task at hand. Since boundary tokens are generated in pairs, stability of lexical boundaries is enforced. Accordingly, stable implementations of certain desirable behaviors, including language-oriented advanced program typography that is robust to user edits, are facilitated using the invented techniques.