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

theorem Th92:
  - x*y = (-x)*y
proof
  thus - x*y = (-1)*(x*y) by Th91
    .= (-1)*x*y by Th66
    .= (-x)*y by Th91;
end;
