theorem Th46:
  for B, R, a st a in B holds B, R |- a
proof
  let B, R, a;
  assume A1: a in B;
  then reconsider B as non empty set;
  reconsider t = a as Element of B by A1;
  set P = <*t*>;
  take P;
  rng P = {t} by FINSEQ_1:38;
  hence thesis by TARSKI:def 1, Th40;
end;
