Two relationship variables in one pattern can bind the same edge #59

Problem

openCypher matches each relationship at most once within one pattern (relationship isomorphism). Here, two relationship variables in the same pattern can bind the same edge.

Reproduction

import pyarrow as pa
from lance_csr import CypherEngine, GraphConfig

config = (
    GraphConfig.builder()
    .node_table("Person", "id")
    .rel_table("KNOWS", "Person", "Person", "src", "dst")
    .build()
)
people = pa.table({"id": [1, 2, 3, 4], "name": ["A", "B", "C", "D"]})
engine = CypherEngine(config, {"Person": people, "KNOWS": pa.table({"src": [1, 3], "dst": [2, 2]})})
engine.execute(
    "MATCH (a:Person)-[r1:KNOWS]->(b:Person)<-[r2:KNOWS]-(c:Person) "
    "RETURN a.name AS a, c.name AS c ORDER BY a, c"
).to_pylist()

This returns 4 rows, including {'a': 'A', 'c': 'A'} and {'a': 'C', 'c': 'C'}, where r1 and r2 are the same edge. openCypher returns 2 rows (A, C and C, A). The result is the same with and without use_csr=True. Named paths are not affected: the path operator from #55 already enforces it.

Suggested fix

For each pair of relationship variables of compatible types in one pattern, add a filter that their identities differ, as the path operator does.