docs: def add_lines(self, lines):
          '''
          Add line vertices and edges to the mesh.
  
          lines: A list of Polyline or Lines objects.
  
          '''
          import numpy as np
          if not lines:
              return
  
          v_lines = np.vstack([l.v for l in lines])
          v_index_offset = np.cumsum([0] + [len(l.v) for l in lines])
          e_lines = np.vstack([l.e + v_index_offset[i] for i, l in enumerate(lines)])
          num_body_verts = self.v.shape[0]
          self.v = np.vstack([self.v, v_lines])
          self.e = e_lines + num_body_verts