New function.
* src/tgba/tgbaproduct.cc, src/tgba/tgbaproduct.hh: Use it to
distinguish acceptance conditions that are identical in both
operands.
* src/tgbatest/explpro4.test: New file.
* src/tgbatest/explpro2.test, src/tgbatest/Makefile.am: Adjust.
