The usual $\left[ abcd... \right]$, places [] delimiters left-and-right, and spans the expression's height.
Contrariwise, I want to more-or-less $\underset{]}{\overset{[}{ abcd... }}$, placing [] "delimiters" top-and-bottom. Moreover (obviously), I want the [] sideways/horizontal, so to speak. And then I also want them wide, spanning the width of that abcd... expression.
Any way to do that, the more straightforward the better (i.e., optimally just some LaTeX directives without special packages, etc)? Thanks.