reserve A for QC-alphabet;
reserve sq for FinSequence,
  x,y,z for bound_QC-variable of A,
  p,q,p1,p2,q1 for Element of QC-WFF(A);

theorem
  p <=> q = p1 <=> q1 implies p = p1 & q = q1
proof
  assume p <=> q = p1 <=> q1;
  then p => q = p1 => q1 by Th2;
  hence thesis by Th11;
end;
