I could take a first crack at it.
With a large production plan like this, we're talking about several thousand aircraft, and you tend to build them in blocks, as was indicated by our colleagues earlier. The idea is that for each block, you're learning as you go what the particular mechanical challenges are, particularly on the F-35. As my colleague, Mr. Perry noted, it's like a flying computer. What are the software challenges? Every time more of these machines enter operation, you are learning about how to fix them and manufacture them, if there was anything on the production lines. It's a normal build cycle.
We see this with ships too, by the way. Often, they can be built in blocks. With this particular block IV, this is where I will often see someone from the joint strike fighter office or Lockheed Martin come in to explain more, because I personally.... Some researchers have a hard time ascertaining specific technical differences about what's going on. One is, in particular, a better engine, perhaps one has better software operates or is better at notifying about the glitches with earlier variants.
That would be my initial answer to it.