worked on error output just a little.

tests/testexpr.jet

@@ -3,4 +3,7 @@
 <set name="test1" value="5 + 5" />
 <set name="test2" expr="expr($[test1])" />
 $[test2]
+<set name="numbers">0123456789</set>
+<set name="lefty" expr="left($[numbers],5)" />
+$[lefty]
 </jet>