3273  static C_char *utf_index(C_word s, C_word i) 
  3274  {
  3275      C_word bv = C_block_item(s, 0);
  3276      if(i == 0) {
  3277          C_set_block_item(s, 2, C_fix(0));
  3278          C_set_block_item(s, 3, C_fix(0));                
  3279          return C_c_string(bv);
  3280      } else if(C_header_size(bv) - 1 == C_unfix(C_block_item(s, 1))) {
  3281          /* len == codepoints */
  3282          C_set_block_item(s, 2, C_fix(i));
  3283          C_set_block_item(s, 3, C_fix(i));        
  3284          return C_c_string(bv) + i;
  3285      }
  3286      return utf_index1(s, i);
  3287  }