[Mathias]

(or, you can skip defining all those variables as intermediate steps):

 
<!-- Each attribute point above one that is allocated by the user costs 4CP * the level, per level -->
<eval index="3" phase="Traits" priority="10000">
  <before name="Calc resLeft"/>
  <after name="Bound trtUser"/><![CDATA[
  hero.child[resCP].field[resSpent].value += (field[trtCPCost].value / 2 * (field[trtUser].value ^ 2 + field[trtUser].value)) - 12
  ]]></eval>