P and D both argued interference before the Board as both claimed they invented certain chemical compounds. P was senior and D was junior. P filed first on October 31, 1980. D sought to prove that he was the first to invent. Under Section 102(g), one who invents first but files second may still be entitled to a patent. D showed that he first acquired the idea of a particular 2-indanyl subgroup in February 1980. D put that in writing. But the realities of the new compound showed (by expert evidence) that one of skilled in the art would be unable to create the compound at that time. D presented evidence that he could make a similar compound as of October 10, 1980. D was eventually able to make the 5-indanyl subgroup with that method in December 1980. D was able to make the 2-indanyl compound in January 1981. The district court gave the verdict to D as it held that D was the first to invent as of October 10, 1980. P appealed.