title: OpenLayers 4 ol.interaction.Modify - how delete last 2 points of Linestring or capture delete operation?
body: <p>It is possible to capture delete event of vertex in OpenLayers?
  The problem is - i have Linestring object on layer with ol.interaction.Modify.</p>
  
  <p>All works great except one thing - it is impossible to delete last 2 vertex of Linestring (i understand why - minimum length of Linestring - 2 vertex, but i need to delete all Linestring in such case).</p>
  
  <p>I can capture event of start modifying and finish modifying, but i can't understand - it was delete operation or not.</p>
  
  <p>May be exists normal solution except modifying code of ОpenLayers?</p>