theorem Th74:
  for L, E, D, g, n ex J, H st J = Polish-expression-hierarchy(L, E, n)
      & H is g-recursive
proof
  let L, E, D, g;
  defpred X[ Nat ] means ex J, H st J = Polish-expression-hierarchy(L, E, $1)
      & H is g-recursive;
  A1: X[ 0 ] by Lm72;
  A2: for n st X[ n ] holds X[ n+1 ] by Lm75;
  thus for n holds X[ n ] from NAT_1:sch 2(A1, A2);
end;
