CountCommon←{
⍝ Can be defined as the 2-train atop
     CountCommon←≢∩
⍝ Literally "count the intersection"
⍝             ≢             ∩
⍝ Example
⍝       'perl' 'weekly' 'challenge' (≢∩) 'raku' 'weekly' 'challenge'
⍝ 2
     ≢⍺∩⍵
 }