Complex sentence: Although Seymour I. Rubinstein was the principal owner of the company, Rob Barnaby was the sole author of the early versions of the program; starting with WordStar 4.0, the program was built on new code written principally by Peter Mierau.
Split into simpler sentences: Although Seymour I. Rubinstein was the principal owner of the company, Rob Barnaby was the sole author of the early versions of the program. But starting with WordStar 4.0, the program was built on new code written principally by Peter Mierau.