Format mathematical intervals, ensuring proper spacing.

When typing an open interval as $]a,b[$, a closing bracket is being used in place of an opening fence and vice versa. This leads to the wrong spacing in, say, $]-a,b[$ or $A\in]a,b[=B$. The package attempts to solve this using:
\interval{a}{b} -> [a,b]
\interval[open]{a}{b} -> ]a,b[
\interval[open left]{a}{b} -> ]a,b]

The package also supports fence scaling and ensures that the enclosing fences will end up having the proper closing and opening types.

TeX maths does not do this job properly.