S = LR

 L = Ls
          |  for s in {a, b}
Rs = sR  /

begin{align*}
S &= LR \
left.
begin{gather*}
L &= L_s\
R_s &= sR
end{gather*}
;;right};;text{for } s in {a, b}\
end{align*}

documentclass{article}
usepackage{amsmath}

begin{document}

begin{align*}
S &= LR \
begin{split}
L &= L_s\
R_s &= sR
end{split}quadBigg},,text{for } s in {a, b}
end{align*}

end{document}