⍸ — Where / Interval index⍸Y lists exact positions, repeated according to their nonnegative integer counts. Higher-rank arguments give coordinate vectors.
⍸0 2 0 1 ⍝ 2x 2x 4x
X⍸Y counts sorted boundaries in X ≤ each query. Uses structural ordering, without tolerance.
10 20 30⍸5 10 25 40 ⍝ 0x 1x 2x 3x