=== Epsilon ===
In physics and machine learning, it is very convenient to use a constant that is
a very small positive number, written in Poi as `eps` or `ε`.
This number has the following property (for all practical purposes):
ε^2 = 0
In the form `(a + b * eps)`, this forms the theory of dual numbers.
Dual numbers are very useful because it lets you "cheat" in ways that seems
unreasonably rewarding and efficient at advanced problem solving.
For example, in matrix algebra, if you have an equation of the form `f(X) = 0`
where `f` is a function of a matrix `X`, you can transform with `det(f(X)) = 0`,
insert dual numbers for coefficients of `X` and tease out matrix solutions.
Poi understands e.g. `(2 + 3 eps)` or `(2 + 3ε)` as `(2 + 3 * ε)`.
Remember to use space in `3 eps` because of `e` in scientific notation.