code: def _vertex_one_color_qubo(x_vars):
      
      Q = {}
      for v in x_vars:
          for color in x_vars[v]:
              idx = x_vars[v][color]
              Q[(idx, idx)] = -1
  
          for color0, color1 in itertools.combinations(x_vars[v], 2):
              idx0 = x_vars[v][color0]
              idx1 = x_vars[v][color1]
  
              Q[(idx0, idx1)] = 2
  
      return Q
docs: For each vertex, it should have exactly one color. Generates
      the QUBO to enforce this constraint.
  
      Notes
      -----
      Does not enforce neighboring vertices having different colors.
  
      Ground energy is -1 * |G|, infeasible gap is 1.