Addressing ATmega registers by number

[system]

void testje() {
  byte maskAnd = 0x00;
  byte maskEor = 0x73;
  byte regNum = 0x04;
  _SFR_IO8( regNum ) = ( _SFR_IO8( regNum ) & maskAnd ) ^ maskEor;
}

Compiles, I can test a bit with it

Thnx