Reading this article and the thought process behind it reminds me of this almost-10-year-old comment I made about how they actually work: https://news.ycombinator.com/item?id=9456444

Selecting an operator loads this value into the accumulator and makes room for the second operand to be typed in.

Here is where I think the first mistake was made; if you observe the effect of repeatedly pressing '=', that obviously does not clear any registers, but merely repeats the last operation.

But there’s more to the story: it’s actually a side effect of a little-known “K-constant” feature that retains one of the previously-entered operands and the operator, and lets you vary the other operand.

I haven't used any 4-function calculator that behaves like this; instead, I get (without clearing):

    2 + 1 0 = 12
    5 =       22
    0 =       32