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Install and launch Karabiner. In the menubar open the menulet Karabiner -> Preferences. Then hit the tab "Misc & Uninstall" and "Open private.xml"

Paste the following lines into the opened xml file:

<?xml version="1.0"?>
<root>
    <item>
      <name>Swap Minus and Alt+Shift+Minus</name>
      <identifier>remap.minus_and_optionlshiftlminus</identifier>
      <autogen>
        __KeyToKey__ 
        KeyCode::MINUS, ModifierFlag::NONE,
        KeyCode::MINUS, ModifierFlag::SHIFT_L | ModifierFlag::OPTION_L
      </autogen>
      <autogen>
        __KeyToKey__
        KeyCode::MINUS, ModifierFlag::SHIFT_L | ModifierFlag::OPTION_L,
        KeyCode::MINUS,
      </autogen>
    </item>
</root>

and save the file.

Return to the tab "Change Key", hit "ReloadXML" and enable "Swap Minus and Alt+Shift+Minus". It should be the first entry listed.

If you are left-handed you may replace _L by _R in the xml file.