Improve precision of boxes when constraints that involve more than one variable

[caballa]

From @caballa on June 30, 2017 16:49

If we have x - y + z >= 5 we can translate into three interval constraints:

• x >= Intervals(y-z) + 5
• y <= Intervals(x+z) - 5
• z >= Intervals(y-x) + 5

where Intervals evaluate each variable to an interval constraint.

Copied from original issue: caballa/crab#19