docs: def count_genes(model):
      """Count the number of distinct genes in model reactions."""
      genes = set()
      for reaction in model.reactions:
          if reaction.genes is None:
              continue
  
          if isinstance(reaction.genes, boolean.Expression):
              genes.update(v.symbol for v in reaction.genes.variables)
          else:
              genes.update(reaction.genes)
  
      return len(genes)