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 Th14:
  W = {F} implies len(W) = len F
proof
  assume
A1: W = {F};
  then
A2: F in W by TARSKI:def 1;
  now
    assume ex x being object st x in W\{F};
    then consider x such that
A3: x in W\{F};
    x in W by A3,XBOOLE_0:def 5;
    hence contradiction by A1,A3,XBOOLE_0:def 5;
  end;
  then
A4: W\{F}={}H by XBOOLE_0:def 1;
  len(W) = (len(W) - len F) + len F .= len(W\{F}) + len F by A2,Th10
    .= 0 + len F by A4,Th13;
  hence thesis;
end;
