theorem Th6:
  H is conjunctive implies H =(the_left_argument_of H) '&' (
  the_right_argument_of H)
proof
  assume
A1: H is conjunctive;
  then ex H1 st H = H1 '&' the_right_argument_of H by Def20;
  hence thesis by A1,Def19;
end;
