clear Mod4
keycode 133 = Alt_L
add Mod4 = Alt_L

The last line causes error to be thrown:

jemt@MacBookPro:~$ xmodmap .xmodmaprc2 
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  118 (X_SetModifierMapping)
  Value in failed request:  0x17
  Serial number of failed request:  14
  Current serial number in output stream:  14