Debussy Preludes: Form?

I'm tutoring some students who'll be taking an AMEB (http://www.ameb.edu.au/) Musicianship (theory, aural, bit of history/general knowledge) exam this year. There are a couple of set works that they have to know and answer questions about, one of which is the Debussy Preludes (just Preludes 1-6 though).

One of the questions that has come up in past exam papers is "what is the form of [insert Debussy Prelude]." I'm at a bit of a loss because I've been studying the preludes and while Dancers of Delphi and the Wind on the Plain are basically ternary form, the others are more free and don't seem to fit into any traditional structures. I'm not really sure what "label" to give them and unfortunately the exam questions seem to be asking for labels, they tend to be of the brief, specific "what is the form" variety, rather than a more open "discuss the form of..." type question.

So is it possible to give a name to the forms of Voiles or Preludes 4-6 or are they too complex/unorthodox? 

Has anyone analysed these pieces?

For example, look at the 6th prelude, Des pas sur la neige (http://conquest.imslp.info/files/imglnks/usimg/8/87/IMSLP59704-PMLP02394-Debussy--Preludes-Livre1--Schirmer-Ed--1stHalf.pdf).

Although these more freely structured preludes could certainly be analysed in different ways, it seems to make a lot of sense as:

A = bars 1-15

A' = bars 16-31

Coda = bars 32-36

So what do you call that? "Sectional form"?  :dunno:  

http://books.google.com/books?id=tMDTSR1V5kwC&pg=PA5&lpg=PA5&dq=debussy+prelude+forms&source=bl&ots=4O0rnc6wLR&sig=EpID-SuzLmJYsj7RZeDb7m7J6cI&hl=en&sa=X&ei=5f4sU_m2BIOGyQGctYHACQ&ved=0CFgQ6AEwBQ#v=onepage&q=debussy%20prelude%20forms&f=false

Maybe this link will help

Thanks, although that book is only Book II of the Preludes, it led me to find this: http://books.google.com.au/books?id=hGu9TsKn1nkC&pg=PP1&lpg=PP1&dq=%22maurice+hinson%22+%22preludes,+book+i%22&source=bl&ots=UiSUlwf37R&sig=OyJPD3ZRKzh668fCyvAUSB92fGo&hl=en&sa=X&ei=xUEuU6buJMbDkwXO2oCABw&ved=0CDkQ6AEwAw#v=onepage&q=%22maurice%20hinson%22%20%22preludes%2C%20book%20i%22&f=false

Des pas sur la neige is here described as "Two part song form." I'm a little puzzled by this "song form" term. Why is it being used for instrumental pieces? Des pas sur la neige is a little bit song-like/lyrical but the term is also applied to Voiles, which seems strange...