Composition is a fundamental problem solving heuristic. In computer science, it primarily appears in program design with concrete objects such as language constructs. It also appears in more abstract forms in higher-level courses. One such form is that of language concatenation in the Computational Models course. This concatenation involves the composition of two specifications of infinite sets (source languages) into a third one, and requires both abstraction and non-deterministic conception. In this paper, we illuminate behaviors of advanced high school students, with such composition. Students who encountered difficulties offered pseudo solutions, which enclosed only "surface" features and observations. We orderly display their solutions, discuss them, and offer suggestions for educators to cope with this phenomenon.