∊ — Enlist / Membership∊Y recursively ravels all nested leaves.
∊(1 2)(3 (4 5)) ⍝ 1 2 3 4 5
X∊Y marks items of X found in Y. Preserves X’s shape and uses tolerant matching.
1 2 3∊2 4 ⍝ 0x 1x 0x
'banana'∊'an' ⍝ 0x 1x 1x 1x 1x 1x