"rescuing" code from the GPL

[Shriramana Sharma <samjnaa@gmail.com>]

Hello. Please inform me kindly if this Q is OT for this list.

I have a question. I will illustrate it by means of a highly simple 
programming situation.

Please look at the following images (each is only 6 KB) to get a 
syntax-highlighted program.

http://h1.ripway.com/jamadagni/01-noqt-nothirdvar.png
http://h1.ripway.com/jamadagni/02-qt-nothirdvar.png
http://h1.ripway.com/jamadagni/03-qt-thirdvar.png
http://h1.ripway.com/jamadagni/04-noqt-thirdvar.png

My question is whether anyone among X, Y and Z in any of the below two 
situations is guilty of copyright infringement as a result of not 
following license conditions?

SITUATION #1:

1. X creates 01-noqt-nothirdvar.cpp and distributes it under the BSDL only.

2. Y modifies this program to use Qt (under the GPL), creating 
02-qt-nothirdvar.cpp, and distributes it under both the BSDL and GPL.

3. Z modifies this program to use a third variable for swapping, 
creating 03-qt-thirdvar.cpp, and distributes it under both the BSDL and GPL.

4. X imports the change from Z, namely using a third variable only, into 
his codebase, creating 04-noqt-thirdvar.cpp, and distributes it under 
the BSDL only.

SITUATION #2:

1. X creates 01-noqt-nothirdvar.cpp and distributes it under the BSDL only.

2. Y modifies this program to use Qt (under the GPL) and to use a third 
variable for swapping, creating 03-qt-thirdvar.cpp, and distributes it 
under both the BSDL and GPL.

3. X imports one change from Y, namely using a third variable only, into 
his codebase, creating 04-noqt-thirdvar.cpp, and distributes it under 
the BSDL only.

Thanks.

Shriramana Sharma.