bug fix on operand operation for left().

tests/testjet.jet

@@ -26,6 +26,9 @@
  <set name="error" value="$[nested]" />
  $[nested]
  <set name="numbers">0123456789</set>
+ $[numbers]
+ <set name="exprnumbers" expr="$[numbers]" />
+ $[exprnumbers]
  <set name="lefty" expr="LEFT($[numbers],5)" />
  lefty=[$[lefty]]
  substring('abcdefg', 1, 3)=[$[theexpr]]