correct sign/1 (#216)

src/prolog/heap_print.rs

@@ -329,7 +329,8 @@ pub fn requires_space(atom: &str, op: &str) -> bool {
             .next()
             .map(|oc| {
                 if ac == '0' {
-                    oc == 'b' || oc == 'x' || oc == 'o' || oc == '\'' || alpha_numeric_char!(oc)
+                    oc == 'b' || oc == 'x' || oc == 'o' || oc == '\''
+                 || oc == '(' || alpha_numeric_char!(oc)
                 } else if alpha_numeric_char!(ac) {
                     oc == '(' || alpha_numeric_char!(oc)
                 } else if graphic_token_char!(ac) {

src/prolog/machine/machine_state_impl.rs

@@ -1381,9 +1381,9 @@ impl MachineState {
         if n.is_positive() {
             Integer::from(1)
         } else if n.is_negative() {
-            Integer::from(0)
-        } else {
             Integer::from(-1)
+        } else {
+            Integer::from(0)
         }
     }