theorem Th97:
  for t, u holds LD-EqClassOf (t => u) = (LD-EqClassOf t) => (LD-EqClassOf u)
proof
  let t, u;
  thus LD-EqClassOf (t => u)
      = (LD-EqClassOf t) '=' (LD-EqClassOf (t '&' u)) by Def93
      .= (LD-EqClassOf t) => (LD-EqClassOf u) by Def92;
end;
