This choice vector tells us:

• bit 0 in hash comes from bit 0 of hash₁(a₁)   ( b_1,0 )
• bit 1 in hash comes from bit 0 of hash₂(a₂)   ( b_2,0 )
• bit 2 in hash comes from bit 0 of hash₃(a₃)   ( b_3,0 )
• bit 3 in hash comes from bit 1 of hash₁(a₁)   ( b_1,1 )
• etc. etc. etc.   (up to as many bits of hashing as required, e.g. 32)