val</span> improve_cost_from_sums : 
  <span><span class="label">on_improve</span>:<span>(<span><a href="../Sum/index.html#type-t">Sum.t</a> <span class="arrow">&#45;&gt;</span></span> <span><a href="../../CostDomain/BasicCost/index.html#type-t">BasicCost.t</a> <span class="arrow">&#45;&gt;</span></span> <span><a href="../../CostDomain/BasicCost/index.html#type-t">BasicCost.t</a> <span class="arrow">&#45;&gt;</span></span> unit)</span> <span class="arrow">&#45;&gt;</span></span>
  <span><span class="label">