Update Iterable.fs

Hello Sergey,
I have put an example that shows how you can reproduce the problem with the ghost array :
- take your existing module ParserDemo2 (file ParserDemo2.fs)
- in line 22, 'sentences' is defined
- suppose that you insert the following code before the 'for loop' of line 39 :
   let length = Seq.length sentences
- it will execute giving you the right length
- if you add another call to Seq like : let length2 = Seq.length sentences 
- then length2 will be 0 <==== that's the PROBLEM
- thereafter the 'for loop' starting in line 39 will not execute properly (no sentence in sentences)

Have a good day,
jliettehk