What the target of a replacement holds once it is done, given what it held before (old): x$a$b <- v reads x$a, sets its b to v, and sets the a of x to the result.
old
x$a$b <- v
x$a
b
v
a
x
What the target of a replacement holds once it is done, given what it held before (
old):x$a$b <- vreadsx$a, sets itsbtov, and sets theaofxto the result.