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 Th8:
  y in still_not-bound_in s '&' h iff y in still_not-bound_in s or
  y in still_not-bound_in h
proof
  still_not-bound_in s '&' h = still_not-bound_in s \/ still_not-bound_in
  h by QC_LANG3:10;
  hence thesis by XBOOLE_0:def 3;
end;
