The 6502 has a group of opcodes which copy bit 5 from the opcode into one of the status flags. 

(I know it's not implemented this way, but it looks as though the bit fields are: 2 bits to select the flag out of `CIVD`, one bit to clear or set, followed by 11000).

See:

```
0001 1000   CLC    Clear carry
0011 1000   SEC    Set carry
0101 1000   CLI    Clear interrupt flag
0111 1000   SEI    Set interrupt flag
1001 1000   CLV    Clear overflow flag
1011 1000   TAY    Transfer A to Y
1101 1000   CLD    Clear decimal flag
1111 1000   SED    Set decimal flag

```

The blatant odd one out here is `TAY`, which takes the place of what looks like should be `SEV`. `SEV` does not exist, but intuitively is exactly as useful as `CLV`, and very easy to implement. 

On the other hand, the group of instructions `TAY`, `TYA`, `TAX`, `TXA` are not really laid out regularly enough that I see any motive for "overriding" what could've been `SEV`.

What happened?