linguistic:voice_change, structural:summarization
Union

If R {\displaystyle R} and S {\displaystyle S} are binary relations over sets X {\displaystyle X} and Y {\displaystyle Y} then R ∪ S = { (x, y) ∣ x R y or x S y } {\displaystyle R\cup S=\{(x,y)\mid xRy{\text{ or }}xSy\}} is the union relation of R {\displaystyle R} and S {\displaystyle S} over X {\displaystyle X} and Y {\displaystyle Y}.

The identity element is the empty relation. For example, ≤ {\displaystyle \leq } is the union of < and =, and ≥ {\displaystyle \geq } is the union of > and =.