What problem does it solve? Understanding what controls a statement or where a variable flows inside a function requires reading GitNexus's opt-in PDG layers (CFG, REACHING_DEF, CDG), whose graph schema and anchoring rules are easy to get wrong. This Skill explains the pdg_query MCP tool's two modes, the corrected Cypher for guard-clause discovery, and the gotchas behind empty or surprising results. ## Core Features & Use Cases - Control-dependence queries: Use pdg_query({ mode: 'controls', target }) to find which predicate blocks guard a statement, including early-return guard clauses flagged with guard: true. - Data-flow queries: Use pdg_query({ mode: 'flows', target, variable? }) to trace def-to-use REACHING_DEF edges for a variable within a function. - Corrected Cypher guidance: Query the single CodeRelation table by its type property with branch sense in reason, since the RFC #567 [:CDG {label:'F'}] form does not run as written. - Use Case: When a pdg_query result comes back empty, check whether the repo was indexed with --pdg; the tool returns a note instead of an error when no PDG layer exists. ## Quick Start Ask the AI to explain what guards a specific statement or trace where a variable flows within a function using the pdg_query tool.
