fix conformity errors

src/prolog/heap_print.rs

@@ -113,6 +113,7 @@ impl HCValueOutputter for PrinterOutputter {
 // the 'classic' display corresponding to the display predicate.
 pub struct WriteqFormatter {}
 
+#[inline]
 fn is_numbered_var(ct: &ClauseType, arity: usize) -> bool {
     arity == 1 && if let &ClauseType::Named(ref name, _) = ct {
         name.as_str() == "$VAR"
@@ -419,10 +420,11 @@ impl<'a, Formatter: HCValueFormatter, Outputter: HCValueOutputter>
             '\u{0c}' => self.outputter.append("\\f"), // UTF-8 form feed
             '\u{08}' => self.outputter.append("\\b"), // UTF-8 backspace
             '\u{07}' => self.outputter.append("\\a"), // UTF-8 alert
-            _ => self.outputter.push_char(c)
+            '\x20' ..= '\x7e' => self.outputter.push_char(c),
+            _ => self.outputter.append(&format!("\\x{:x}", c as u32))
         };
     }
-
+    
     fn print_constant(&mut self, c: Constant, op: &Option<DirectedOp>) {
         match c {
             Constant::Atom(ref atom, Some(fixity)) => {