class_docstr: An RPSO class, inherited from Optimizer.
  
  This is the designed class to define RPSO-related
  variables and methods.
  
  References:
      M. Roder, G. H. de Rosa, L. A. Passos, A. L. D. Rossi and J. P. Papa.
      Harnessing Particle Swarm Optimization Through Relativistic Velocity.
      IEEE Congress on Evolutionary Computation (2020).
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docstr: Array of masses.
docstr_tok: ["Array","of","masses","."]
code: def mass(self) -> np.ndarray:
      """Array of masses."""
  
      return self._mass
  
imports: import copy
  import time
  from typing import Any, Dict, List, Optional
  import numpy
  import opytimizer.math.random
  import opytimizer.utils.constant
  import opytimizer.utils.exception
  from opytimizer.core import Optimizer
  from opytimizer.core.agent import Agent
  from opytimizer.core.function import Function
  from opytimizer.core.space import Space
  from opytimizer.utils import logging
label_desc: Use image node_id 2 for calling the RPSO obj's underlying member method code with example usage: obj.mass() and returns: self