bool contains(cstring_span<> word) const { return symbols_.find(word) != kNullIndex; }
 bool contains(std::string const& word) const { return symbols_.find(word) != kNullIndex; }
 Sym sym(std::string const& word) const {
   SymInt const i = symbols_.find(word);
   return i == kNullIndex ? NoSymbol : symForIndex(i);
 }
 Sym sym(cstring_span<> word) const {
   SymInt const i = symbols_.find(word);
   return i == kNullIndex ? NoSymbol : symForIndex(i);
 }
 SymInt index(std::string const& word) const { return symbols_.find(word); }