(1.3.1, 24)

Knuth asks:

If we represent a subset S of {0, 1, ..., 63} by the bit vector

([0 ∈ S], [1 ∈ S], ..., [63 ∈ S]),

the bitwise operations ∧ and ∨ correspond respectively to 
set intersection  (S ∩ T) and set union (S ∪ T).

Which bitwise operation corresponds to set difference (S\T)?

The set S\T is characterised by the predicate

S ∧ ¬T

.
Hence the required operation is ANDN.