∪ — Unique / Union∪Y keeps the first of each matching major cell. Uses tolerant matching.
∪3 1 3 2 1 ⍝ 3 1 2
X∪Y appends items of Y absent from X. Order and repetitions are retained.
1 2∪2 3 ⍝ 1 2 3
1 1∪2 2 ⍝ 1 1 2 2