The result R is the left argument X.
42⊣'abc' 1 2 3 42
Note that when ⊣ is applied using reduction, the derived function selects the first sub-array of the array along the specified dimension. This is implemented as an idiom.
⊣/1 2 3 1 mat←↑'scent' 'canoe' 'arson' 'rouse' 'fleet' ⊣⌿mat ⍝ first row scent ⊣/mat ⍝ first column scarf
⊣/[2]2 3 4⍴⍳24 ⍝ first row from each plane 1 2 3 4 13 14 15 16
Similarly, with expansion:
⊣\mat sssss ccccc aaaaa rrrrr fffff ⊣⍀mat scent scent scent scent scent