Format mathematical intervals, ensuring proper spacing
Description
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.
