reserve k,n,n1,m,m1,m0,h,i,j for Nat,
  a,x,y,X,X1,X2,X3,X4,Y for set;
reserve L,L1,L2 for FinSequence;
reserve F,F1,G,G1,H for LTL-formula;
reserve W,W1,W2 for Subset of Subformulae H;
reserve v for LTL-formula;
reserve N,N1,N2,N10,N20,M for strict LTLnode over v;
reserve w for Element of Inf_seq(AtomicFamily);
reserve R1,R2 for Real_Sequence;

theorem Th17:
  len(W) >= 0
proof
  now
    per cases;
    suppose
      W = {}H;
      hence thesis by Th13;
    end;
    suppose
      W <> {}H;
      hence thesis by Th16;
    end;
  end;
  hence thesis;
end;
