induction n using recBase2
n : Nat
hpos : n > 0
⊢ log2' n < n
case zero
hpos : 0 > 0
⊢ log2' 0 < 0

case one
hpos : 1 > 0
⊢ log2' 1 < 1

case div2
n✝ : Nat
a✝ : n✝ / 2 + 1 > 0 → log2' (n✝ / 2 + 1) < n✝ / 2 + 1
hpos : n✝ + 2 > 0
⊢ log2' (n✝ + 2) < n✝ + 2