In a paper by Camescasse et al.: "Bistable buckled beam: Elastica modeling and analysis of static actuation" in equation 23, they use a (to me) unknown notation for which i can't find any good resources or other examples from related papers.
$$ \left\{ \frac{d\mathcal{U}}{ds} \right\}_{G(s)} = \frac{d}{ds} \genfrac{\{}{\}}{0pt}{}{\vec{q}(s)}{\vec{p}(s)}_{G(s)} $$
I don't know what concept they express, or why there are two vectors stacked on top of each other on the right side of the equation.
They use that notation later on as well in an integral, in equation (28), rendering me unable to follow the calculations.