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>

tests/testjet.jet

@@ -19,9 +19,9 @@
  <set name="division" expr="5/3" />
  <set name="divisor" value="8" />
  <set name="nested" expr="(2*(4+4)/$[divisor])*32" />
- $[nested]
+ $[$nested]
  <set name="numbers">0123456789</set>
- <set name="lefty" expr="LEFT('$[numbers]',5)" />
+ <set name="lefty" expr="LEFT($[numbers],5)" />
  lefty=[$[lefty]]
  substring('abcdefg', 1, 3)=[$[theexpr]]
  5+3=($[addition])