[dragonwriter]

> Say x = 4.0 ± 1.0. What is x / x?

> It should be x / x = 1.0 ± 0.0, but interval arithmetic will give you [3/5, 5/3].

This is the “dependency problem” which is eliminated in many cases, and mitigated in others, by rewriting so identical (vs. merely in components) values appear only once; when you might need a numerical answer at one point (if possible) but to use the value in further computations, this can be done by storing and manipulating values symbolically and extracting numerical results as needed.