English
testBit distributes over bitwise AND: testBit (m &&& n) k = (testBit m k) ∧ (testBit n k).
Русский
testBit распределяется через побитовое AND: testBit (m &&& n) k = testBit m k ∧ testBit n k.
LaTeX
$$$$ \\text{testBit}(m\\,\\&\\&\\&\\,n)\\,k = (\\text{testBit}\\,m\\,k) \\land (\\text{testBit}\\,n\\,k) $$$$
Lean4
theorem testBit_land : ∀ m n k, testBit (m &&& n) k = (testBit m k && testBit n k) :=
testBit_bitwise rfl