code: def sub_state_by_gene_name(self, *gene_names: str) -> 'State':
          
          return State({gene: state for gene, state in self.items() if gene.name in gene_names})
docs: Create a sub state with only the gene passed in arguments.
  
          Example
          -------
  
          >>> state.sub_state_by_gene_name('operon')
          {operon: 2}
          >>> state.sub_state_by_gene_name('mucuB')
          {mucuB: 0}