Catch division by zero when simplifying powers.

Author: Kasper Peeters

core/Algorithm.cc

@@ -337,7 +337,12 @@ void Algorithm::propagate_zeroes(post_order_iterator& it, const iterator& topnod
 		// Removing happens in the next step.
 		}
 	else if(*walk->name=="\\pow") {
-		if(tr.index(it)==0) { // the argument
+		if(tr.index(it)==0) { // the argument is zero
+			sibling_iterator arg=it;
+			++arg;
+			if(*arg->multiplier<0) 
+				throw RuntimeException("Division by zero encountered.");
+			
 			walk->multiplier=rat_set.insert(0).first;
 			it=walk;
 			propagate_zeroes(it, topnode);

core/Cleanup.cc

@@ -149,11 +149,14 @@ namespace cadabra {
 			return true;
 			}
 
+		if(*arg->multiplier==0) { // 0**positive = 0, 0**negative=undefined
+			if(*exp->multiplier<0)
+				throw RuntimeException("Division by zero encountered.");
+			zero(it->multiplier);
+			return true;
+			}
+		
 		if(*arg->name=="1") {
-			if(*arg->multiplier==0) { // 0**anything = 0
-				zero(it->multiplier);
-				return true;
-				}
 			if(*arg->multiplier==1) { // 1**anything = 1
 				one(it->multiplier);
 				tr.erase_children(it);

tests/numerical.cdb

@@ -96,4 +96,23 @@ def test06():
 
 test06()
 
-        
+def test07():
+    try:
+       ex:=1/0;
+       assert(1==0)
+    except RuntimeError as err:
+       print("Test 07a passed")
+    try:
+       ex:=1/a;
+       substitute(ex, $a->0$)
+       assert(1==0)
+    except RuntimeError as err:
+       print("Test 07b passed")
+    try:
+       ex:=1/(a-b);
+       substitute(ex, $a->b$)
+       assert(1==0)
+    except RuntimeError as err:
+       print("Test 07c passed")
+
+test07()