theorem
  [q,t,K,f] in SepQuadruples p implies not x.t in f.:K
proof
  assume
A1: [q,t,K,f] in SepQuadruples p;
  assume x.t in f.:K;
  then t < t & t <= t by A1,Th39,QC_LANG1:22;
  hence contradiction by QC_LANG1:25;
end;
