theorem Th90:
  for t st LD-EqClassOf t is LD-provable holds t is LD-provable
proof
  let t;
  set x = LD-EqClassOf t;
  assume x is LD-provable;
  then consider u such that A1: LD-EqClassOf u = x and A2: u is LD-provable;
  u LD-= t by A1, Th80;
  then u '=' t is LD-provable;
  hence thesis by A2, Th61;
end;
