docs: def valid(self,individuals=None,F=None):
          """returns the sublist of individuals with valid fitness."""
          if F:
              valid_locs = self.valid_loc(F)
          else:
              valid_locs = self.valid_loc(self.F)
  
          if individuals:
              return [ind for i,ind in enumerate(individuals) if i in valid_locs]
          else:
              return [ind for i,ind in enumerate(self.pop.individuals) if i in valid_locs]