Adds one edge, folding a repeat of a (source, target, type) triple
already present into the surviving edge instead of dropping it outright.
The first instance's express id becomes relationshipId; later repeats
are kept in _shadowedRelIds so deleting one of several IfcRel*
instances that named the same pair doesn't make the connection
disappear while a sibling instance still exists (#3782 review).
Adds one edge, folding a repeat of a
(source, target, type)triple already present into the surviving edge instead of dropping it outright. The first instance's express id becomesrelationshipId; later repeats are kept in_shadowedRelIdsso deleting one of severalIfcRel*instances that named the same pair doesn't make the connection disappear while a sibling instance still exists (#3782 review).