The function evaluating the parse tree can be simplified if you eliminate elsif at compile time. Keep in mind that

if cond1 then
   action1
elsif cond2 then
   action2
elsif cond3 then
   action3
else 
   action4
endif
[download]

is equivalent to

if cond1 then
   action1
else
   if cond2 then
      action2
   else
      if cond3 then
         action3
      else
         action4
      endif
   endif
endif
[download]

except the latter is easier to evaluate because the parse tree is simply

[
   if => cond1, action1, [
      if => cond2, action2, [
         if => cond3, action3, action4
      ]
   ]
]
[download]

See my reply below (under the heading "The following returns a nice tree for expr to evaluate") for an implementation.