bits1.size() == 64 possible bit positions.

The bits that are on in bits1 are {0, 2}
The bits that are on in bits2 are {1, 2}

After: bits1 AND bits2...
bits1: bit 0 is set?  false
bits1: bit 1 is set?  false
bits1: bit 2 is set?  true
bits1 = {2}

After: bits1 OR bits2...
bits1: bit 0 is set?  true
bits1: bit 1 is set?  true
bits1: bit 2 is set?  true
bits1 = {0, 1, 2}

After: bits1 XOR bits2...
bits1: bit 0 is set?  true
bits1: bit 1 is set?  true
bits1: bit 2 is set?  false
bits1 = {0, 1}

Before any bits are cleared, bits1 = {0, 2}
After clearing bit 0,        bits1 = {2}