bug fix on operand operation for left().

tests/testexpr.jet

@@ -2,8 +2,8 @@
 <jet name1="localname" filterblanklines="true" trimlines="true">
 <set name="test1" value="5 + 5" />
 <set name="test2" expr="expr($[test1])" />
-$[test2]
+$[test2]=10
 <set name="numbers">0123456789</set>
 <set name="lefty" expr="left($[numbers],5)" />
-$[lefty]
+$[lefty]=01234
 </jet>