BitSum←{
⍝ ⍵: List of positive integers
⍝ ←: Sum of differing bits between binary
⍝      representations of all unique pairs in ⍵
     BSP←{+⌿≠⌿⍉2⊥⍣¯1⊢⍺ ⍵}   ⍝ Bit Sum of Pairs
     bs←+/,∘.BSP⍨⍵          ⍝ The bit sum of all pairs in ⍵
     bs÷2                   ⍝ Outer product ∘.f double counts
 }