Does the function return two values?
It returns a pair or 2D vector; the authors have opted for [a;b] over (a,b) to improve legibility.
The other line I had trouble with was this one: min s' (is element of) pred(s) (g(s'), c(s', s)). I don't get that one at all. The s' in pred() with the smallest g or c?
I don't see this line. I do see min s' (is element of) pred(s) (g(s') + c(s', s)) - i.e. the minimum of the expression on the right over all predecessors of s.
EDIT: slightly clearer wording.