reserve x,y,z,w for ExtReal,
  r for Real;
reserve f,g for ExtReal;
reserve t for ExtReal;

theorem
  x is real implies x * (y - z) = x * y - x * z
proof
  assume x is real;
  then x * (y - z) = x * y + x * (-z) by Th95
    .= x * y + -(x * z) by Th92;
  hence thesis;
end;
