Here's an awk script that attempts to set difference of two files based on their first column:

```
BEGIN{
    OFS=FS="\t"
    file = ARGV[1]
    while (getline < file)
        Contained[$1] = $1
    delete ARGV[1]
    }
$1 not in Contained{
    print $0
}

```

Here is TestFileA:

```
cat
dog
frog

```

Here is TestFileB:

```
ee
cat
dog
frog

```

However, when I run the following command:

```
gawk -f Diff.awk TestFileA TestFileB

```

I get the output just as if the script had contained "in":

```
cat
dog
frog

```

While I am uncertain about whether "not in" is correct syntax for my intent, I'm very curious about why it behaves exactly the same way as when I wrote "in".