Fix vary acting on a zero expression.

Author: Kasper Peeters

core/algorithms/vary.cc

@@ -18,6 +18,8 @@ vary::vary(const Kernel& k, Ex& tr, Ex& args_)
 
 bool vary::can_apply(iterator it)
 	{
+	if(it->is_zero()) return false;
+	
 	if(*it->name=="\\prod") return true;
 	if(*it->name=="\\commutator") return true;
 	if(*it->name=="\\anticommutator") return true;

tests/vary.cdb

@@ -169,10 +169,22 @@ def test14():
     \delta{#}::Accent.
     \Vt{#}::LaTeXForm("{\tilde{V}}").
     ex:=\int{\Vt_{\mu}\Vt^{\mu}}{x};
-    vary(_,$\Vt_{\mu}->\delta{\Vt_{\mu}}$);
+    vary(_,$\Vt_{\mu}->\delta{\Vt_{\mu}}$)
     tst:= \int{\delta{\Vt_{\mu}} \Vt^{\mu} + \Vt_{\mu} \delta{\Vt^{\mu}}}{x} - @(ex);
-    display(tst)
     assert(tst==0)
     print("Test 14 passed")
 
 test14()
+
+def test15():        
+    __cdbkernel__=create_scope()
+    ex:=A;
+    vary(_, $A -> a$)
+    assert(ex==$a$)
+    print("Test 15a passed")        
+    ex:=0;        
+    vary(_, $A -> a$)
+    assert(ex==0)
+    print("Test 15b passed")
+
+test15()