def angle( vertex_index )
      return nil if(vertex_index < 0 || vertex_index > 2)
      vert1 = self.vertices[vertex_index]
      vert2 = self.vertices[(vertex_index+1)%3]
      vert3 = self.vertices[(vertex_index+2)%3]
      vec1 = vert2 - vert1
      vec2 = vert3 - vert1
      vec1.angle(vec2)
    end
def angle( vertex_index )
      return nil if(vertex_index < 0 || vertex_index > 2)
      vert1 = self.vertices[vertex_index]
      vert2 = self.vertices[(vertex_index+1)%3]
      vert3 = self.vertices[(vertex_index+2)%3]
      vec1 = vert2 - vert1
      vec2 = vert3 - vert1
      vec1.angle(vec2)
    end
[Input]
  _vertex_index_ should be 0..2.
 [Output]
  return angle as Numeric(radian).