From J Wiki
[x] u@v y Atop Conjunction [x] u@v y Atop Conjunction
Forms the composition (u@v) of verbs u and v .
The resulting verb is applied independently to each cell of the argument(s).
- Operand u is always executed monadically on the result of each application of v
- Operand v is executed either monadically or dyadically depending whether u@v has been called monadically or dyadically