- Improved code for new preferred order swapping of substitutions. Also

included more comments.

src/mgu.c

@@ -139,24 +139,22 @@ termMguTerm (Term t1, Term t2)
 	}
     }
 
-  /* symmetrical tests for single variable */
   /*
-   * I broke symmetry later, for the special case where both are unbound
-   * variables that will be connected, and I want to give one priority over the
-   * other for readability.
+   * Distinguish a special case where both are unbound variables that will be
+   * connected, and I want to give one priority over the other for readability.
+   *
+   * Because t1 and t2 have been deVar'd means that if they are variables, they
+   * are also unbound.
    */
-  if (realTermVariable (t2))
+
+  if (realTermVariable (t1) && realTermVariable (t2) && goodsubst (t1, t2))
     {
-      if (termSubTerm (t1, t2) || !goodsubst (t2, t1))
-	return MGUFAIL;
-      else
-	{
-	  // Symmetry break starts here
-	  if (isTermVariable (t1))
-	    {
-	      // Both are variables. Decide.
-	      // The plan: t2->subst will point to t1. Is that desired?
-	      if (preferSubstitutionOrder (t1,t2))
+      /* Both are unbound variables. Decide.
+       *
+       * The plan: t1->subst will point to t2. But maybe we prefer the other
+       * way around?
+       */
+      if (preferSubstitutionOrder (t2, t1))
 	{
 	  Term t3;
 
@@ -165,8 +163,22 @@ termMguTerm (Term t1, Term t2)
 	  t1 = t2;
 	  t2 = t3;
 	}
+      t1->subst = t2;
+#ifdef DEBUG
+      showSubst (t1);
+#endif
+      return termlistAdd (NULL, t1);
     }
 
+  /* symmetrical tests for single variable.
+   */
+
+  if (realTermVariable (t2))
+    {
+      if (termSubTerm (t1, t2) || !goodsubst (t2, t1))
+	return MGUFAIL;
+      else
+	{
 	  t2->subst = t1;
 #ifdef DEBUG
 	  showSubst (t2);