reserve A for QC-alphabet;
reserve X,T for Subset of CQC-WFF(A);
reserve F,G,H,p,q,r,t for Element of CQC-WFF(A);
reserve s,h for QC-formula of A;
reserve x,y for bound_QC-variable of A;
reserve f for FinSequence of [:CQC-WFF(A),Proof_Step_Kinds:];
reserve i,j for Element of NAT;

theorem
  not x in still_not-bound_in p implies ( p '&' Ex(x,q) is valid iff Ex(
  x,p '&' q ) is valid )
proof
  assume not x in still_not-bound_in p;
  then (p '&' Ex(x,q)) <=> Ex(x,p '&' q) is valid by Th72;
  hence thesis by Lm15;
end;
