code: func (vs *VertexSlice) Slice(i, j int) *VertexSlice {
  	if i < 0 || j < i || j > vs.va.cap {
  		panic("failed to slice vertex slice: index out of range")
  	}
  	return &VertexSlice{
  		va: vs.va,
  		i:  vs.i + i,
  		j:  vs.i + j,
  	}
  }
docs: // Slice returns a sub-slice of this VertexSlice covering the range [i, j) (relative to this
  // VertexSlice).
  //
  // Note, that the returned VertexSlice shares an underlying vertex array with the original
  // VertexSlice. Modifying the contents of one modifies corresponding contents of the other.