reserve n,m,k,k1,k2,i,j for Nat;
reserve x,y,z for object,X,Y,Z for set;
reserve A for Subset of X;
reserve B,A1,A2,A3 for SetSequence of X;
reserve Si for SigmaField of X;
reserve S,S1,S2,S3 for SetSequence of Si;

theorem
  Intersection B c= (inferior_setsequence B).n
proof
  0 <= n by NAT_1:2;
  then (inferior_setsequence B).0 c= (inferior_setsequence B).n by PROB_1:def 5
;
  hence thesis by Th17;
end;
