Today I learnt a bit about `tr` command.

But I was stuck at understanding the difference between `-c` and `-C`.

On the manual, it said:

```
 -C      Complement the set of characters in string1, that is ``-C ab'' includes every character except for `a' and `b'.

 -c      Same as -C but complement the set of values in string1.

```

I'm not quite understand what does `set of values in string1` of `-c` option mean.  

I thought it may treat string 1 `"ab"` as a whole and will escape single `a` and `b`.  

So I did an experiment:

```
⇒  echo "ab_a_b" | tr -C 'ba' 'c'
abcacbc%                                                                                                                                                                             
⇒  echo "ab_a_b" | tr -c 'ba' 'c'
abcacbc%

```

Things didn't match my expectation!  

So, what's the difference between `-C` and `-c` in `tr` command?

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Software Version: BSD 2004 on OSX10.10