theorem Th21:
  (p '&' q).x = (p.x) '&' (q.x)
proof
  set pq = p '&' q;
A1: p '&' q is conjunctive by QC_LANG1:def 20;
  then the_left_argument_of pq = p & the_right_argument_of pq = q by
QC_LANG1:def 25,def 26;
  hence thesis by A1,Th20;
end;
