A piecewise map is honest only when the branch choices are checked. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A branch choice is part of the source
Starting at three quarters uses the high branch and lands at one half.
2(1−43)=21
The equality boundary is explicit
At one half the low-or-equal branch gives 1, then the high branch gives 0.