reserve p,q,r for FinSequence;
reserve u,v,x,y,y1,y2,z for object, A,D,X,Y for set;
reserve i,j,k,l,m,n for Nat;

theorem Th58:
  len(p - A) <= len p
proof
  len(p - A) = len p - card(p " A) by Th57;
  hence thesis by XREAL_1:43;
end;
