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 Th4:
  the_left_argument_of (p '&' q) = p & the_right_argument_of (p '&' q) = q
proof
  p '&' q is conjunctive;
  hence thesis by QC_LANG1:def 25,def 26;
end;
