anchor: Contact:getChildren
  Gets the child indices of the shapes of the two colliding fixtures. For ChainShapes, an index of 1 is the first edge in the chain. Used together with Fixture:rayCast or ChainShape:getChildEdge.
  indexA, indexB = Contact:getChildren()
  indexA number The child index of the first fixture's shape.
  indexB number The child index of the second fixture's shape.