Programming :: Extract Extra Packages In A Separate File?

I have two files containing list of packages using

Code:

dpkg --get-selections > file-name
command.
package-a.txt

[code]......

Now I would like to create a third file which contains only those packages which are present in package-a.txt but NOT in package-b.txt. The file should look like this:

Code:

package2
package4

Note: The world "install" is also to be removed for all packages. Using diff command I could get something like this:

Code:

temp# diff -yb package-a.txt package-b.txt | grep "<"
package2 install <
package4 install <
<
temp#

But not sure how to remove instances of "install" and "<".