reserve k,n,m for Nat,
  a,x,X,Y for set,
  D,D1,D2,S for non empty set,
  p,q for FinSequence of NAT;
reserve F,F1,G,G1,H,H1,H2 for LTL-formula;
reserve sq,sq9 for FinSequence;
reserve L,L9 for FinSequence;
reserve j for Nat;
reserve j1 for Element of NAT;
reserve V for LTLModel;
reserve Kai for Function of atomic_LTL,the BasicAssign of V;
reserve f,f1,f2 for Function of LTL_WFF,the carrier of V;
reserve BASSIGN for non empty Subset of ModelSP(Inf_seq(S));
reserve t for Element of Inf_seq(S);
reserve f,g for Assign of Inf_seqModel(S,BASSIGN);

theorem Th61:
  t |= f 'or' g iff (t|= f or t|= g)
proof
  t |= f 'or' g iff t|= 'not' ('not'(f) '&' 'not'(g)) by Def54;
  then t |= f 'or' g iff not t|= 'not'(f) '&' 'not'(g) by Th57;
  then t |= f 'or' g iff not t|= 'not'(f) or not t|= 'not'(g) by Th58;
  hence thesis by Th57;
end;
