asterix-graphix/src/main/java/org/apache/asterix/graphix/lang/rewrite/lower/EnvironmentActionFactory.java [508:519]:
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        List<EdgeIdentifier> edgeElementIDs = edgePatternExpr.generateIdentifiers(graphIdentifier);
        if (edgeElementIDs.size() != 1) {
            throw new CompilationException(ErrorCode.COMPILATION_ILLEGAL_STATE, "Found non-canonical edge pattern!");
        }
        EdgeIdentifier edgeIdentifier = edgeElementIDs.get(0);

        // ...and only one vertex identifier (given that we only have one label).
        List<VertexIdentifier> vertexElementIDs = vertexPatternExpr.generateIdentifiers(graphIdentifier);
        if (vertexElementIDs.size() != 1) {
            throw new CompilationException(ErrorCode.COMPILATION_ILLEGAL_STATE, "Found non-canonical vertex pattern!");
        }
        VertexIdentifier vertexIdentifier = vertexElementIDs.get(0);
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asterix-graphix/src/main/java/org/apache/asterix/graphix/lang/rewrite/lower/EnvironmentActionFactory.java [555:566]:
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        List<EdgeIdentifier> edgeElementIDs = edgePatternExpr.generateIdentifiers(graphIdentifier);
        if (edgeElementIDs.size() != 1) {
            throw new CompilationException(ErrorCode.COMPILATION_ILLEGAL_STATE, "Found non-canonical edge pattern!");
        }
        EdgeIdentifier edgeIdentifier = edgeElementIDs.get(0);

        // ...and only one vertex identifier (given that we only have one label).
        List<VertexIdentifier> vertexElementIDs = vertexPatternExpr.generateIdentifiers(graphIdentifier);
        if (vertexElementIDs.size() != 1) {
            throw new CompilationException(ErrorCode.COMPILATION_ILLEGAL_STATE, "Found non-canonical vertex pattern!");
        }
        VertexIdentifier vertexIdentifier = vertexElementIDs.get(0);
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