theorem Th3:
  p is conjunctive implies p = (the_left_argument_of p) '&'
  the_right_argument_of p
proof
  given p1,q1 such that
A1: p = p1 '&' q1;
A2: p is conjunctive by A1;
  then p1 = the_left_argument_of p by A1,QC_LANG1:def 25;
  hence thesis by A1,A2,QC_LANG1:def 26;
end;
