Catenate First R←X⍪[K]Y
The form R←X⍪Y implies catenation along the first axis whereas the form R←X,Y implies catenation along the last axis (columns). See Catenate/Laminate.
The form R←X⍪Y implies catenation along the first axis whereas the form R←X,Y implies catenation along the last axis (columns). See Catenate/Laminate.