reserve A for QC-alphabet;
reserve i,j,k for Nat;
reserve f for Substitution of A;
reserve x,y for bound_QC-variable of A;
reserve a for free_QC-variable of A;
reserve p,q for Element of QC-WFF(A);
reserve l,l1,l2,ll for FinSequence of QC-variables(A);
reserve r,s for Element of CQC-WFF(A);

theorem Th24:
  (All(x,p)).x = All(x,p)
proof
  set q = All(x,p);
A1: q is universal by QC_LANG1:def 21;
  then bound_in q = x by QC_LANG1:def 27;
  hence thesis by A1,Th22;
end;
