theorem Th108:
  for a being SortSymbol of J
  st x in X.a & x nin (vf A).a & \for(x,A\impB) in G
  holds A\imp\for(x,B) in G
  proof let a be SortSymbol of J;
    assume
A1: x in X.a & x nin (vf A).a & \for(x,A\impB) in G; then
    \for(x,A\impB)\imp(A\imp\for(x,B)) in G by Def39;
    hence thesis by Def38,A1;
  end;
