We describe gtutr, an interactive feedback system designed to assist students in developing context-free grammars and corresponding ANTLR parsers. It intelligently controls students' access to a large test suite for the target language. After each submission, gtutr analyzes any failing tests and uses the Needleman-Wunsch sequence alignment algorithm over the tests' rule traces to identify and eliminate similar failing tests. This reduces the redundancy in the feedback
given to the students and prevents them from being overloaded. gtutr uses simple gamification to encourage independent problem solving by students: it gives as little information as possible, and students need to prompt the system for further details such as failing tests similar to or different from already seen tests, or even for hints about rules that are the most likely to contain faults. It tracks the students' information requests and uses this to attenuate marks following an instructor-set penalty schema. The system also visualizes test outcomes over multiple submissions, helping students to keep track of the effects of their changes as their grammar development progresses.