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);
reserve s,t for bound_QC-variable of A;
reserve F,G,H,H1 for Element of QC-WFF(A);

theorem Th31:
  the_left_side_of(F <=> G) = F & the_right_side_of(F <=> G) = G &
the_left_argument_of(F <=> G) = F => G & the_right_argument_of(F <=> G) = G =>
  F
proof
  the_antecedent_of(F => G) = F & the_consequent_of(F => G) = G by Th30;
  hence thesis by Th4;
end;
