Hi Jon,
I never understood "interpolating a
register" to have any relation to
the definition of interpolate that I learned in math.
The first definition makes sense of it:
1. (transitive, intransitive) To introduce (something) between other
things; especially to insert (possibly spurious) words into a text.
in verse 74, the second line is clearly interpolated
2. (mathematics) To estimate the value of a function between two
tabulated points.
3. (computing) During the course of processing some data, and in
response to a directive in that data, to fetch data from a different
source and process it in-line along with the original data.
―
https://en.wiktionary.org/wiki/interpolate
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Cheers, Ralph.