theorem Th105:
  \ex(x,A)\iff\not\for(x,\notA) in G
  proof
    \not\ex(x,A)\iff\for(x,\notA) in G by Def39; then
    \not\not\ex(x,A)\iff\not\for(x,\notA) in G by Th94;
    hence \ex(x,A)\iff\not\for(x,\notA) in G by Th95;
  end;
