A nice intro to the concepts of array programming is https://en.wikipedia.org/wiki/Array_programming
Don't feel bad about trial and error; my first go at this (and I have the dubious privilege of being current PDL maintainer) assumed (wrongly) that dupN would be suitable for both sides of the multiplication; furthermore, it took me a little while to figure out inflateN, even though, annoyingly, it is nearly identical to dupN except for swapping the two parts of the slice argument.
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