theorem Th106:
  \for(x,A)\iff\not\ex(x,\notA) in G
  proof
    \ex(x,\notA)\iff\not\for(x,A) in G by Def39; then
    \not\for(x,A)\iff\ex(x,\notA) in G by Th90; then
    \not\not\for(x,A)\iff\not\ex(x,\notA) in G by Th94;
    hence \for(x,A)\iff\not\ex(x,\notA) in G by Th95;
  end;
