Text spanner text in the middle

[Andrew Bernard]

Hello Ponderers,

I need a custom text spanner with text centred in the middle of the spanner. I know you can provide text at the left or the right with the bound-details alist, but how would one go about making a spanner with text in the middle, similar to a tuplet bracket which has the number in the centre of the span?

So, like this:

[==== x3 ====]

Andrew