ADDED: '$first_non_octet'/2, for much more efficient domain checks in library(crypto).

'$first_non_octet'(Cs, C) <=> C is the leftmost character in the
string Cs whose character code is not in 0..255.

src/machine/system_calls.rs

@@ -5340,6 +5340,18 @@ impl MachineState {
                 let cstr = self.heap.put_complete_string(&string);
                 (self.unify_fn)(self, self[temp_v!(3)], cstr);
             }
+            &SystemClauseType::FirstNonOctet => {
+                for c in self.heap_pstr_iter(self[temp_v!(1)]).to_string().chars() {
+                    if c as u32 > 255 {
+                        let chars = clause_name!(String::from(c.to_string()), self.atom_tbl);
+                        let non_octet = self.heap.to_unifiable(HeapCellValue::Atom(chars, None));
+                        (self.unify_fn)(self, self[temp_v!(2)], non_octet);
+                        return return_from_clause!(self.last_call, self);
+                    }
+                }
+                self.fail = true;
+                return Ok(());
+            }
             &SystemClauseType::LoadHTML => {
                 let string = self.heap_pstr_iter(self[temp_v!(1)]).to_string();
                 let doc = select::document::Document::from_read(string.as_bytes()).unwrap();