theorem Th72:
  w |= v iff w |= *('X' init v)
proof
  set N = init v;
  set M = 'X' N;
A1: *M = {v} by Lm33;
A2: w |= *M implies w |= v
  proof
    assume
A3: w |= *M;
    v in *M by A1,TARSKI:def 1;
    hence thesis by A3;
  end;
  w |= v implies w |= *M
  by A1,TARSKI:def 1;
  hence thesis by A2;
end;
