theorem Th7:
  H is disjunctive implies H =(the_left_argument_of H) 'or' (
  the_right_argument_of H)
proof
  assume
A1: H is disjunctive;
  then ex H1 st H = H1 'or' the_right_argument_of H by Def20;
  hence thesis by A1,Def19;
end;
