Impress has a bracket pair shape shape within the symbol shapes, and that's a good thing. The thing is, those are supposed to be square brackets: That's what the button icon shows, and that's the more typical kind of bracket where the left and right bracket edges are straight lines ... and yet, the brackets have strongly rounded corners: large quarter-circles ending the bracket, with no horizontal straight line going further inwards.

There's a drag control for how large the quarter-circles are - which again is good. But another control is missing, so that together one can control both the in-protrusion extent, and its roundness; and specifically, be able to create a perfectly-sharp-corner pair of brackets if one wants to.