List Subsets

[Simon]

Hi,

I'm hoping you could show me examples of how a functional/declarative
language could be used to consicely describe resticted subsets of elements.
I'm looking for a 'specification' style definition so any ideas/input would
be very welcome.

Thanks for your time,
Simon.
--
Say I have a list of items which is considered ordered...

['iA', 'iB', 'iC', 'iD', 'iE', 'iF']

(
  which could also be enumerated if needed...
  [(0,'iA'), (1,'iB'), (2,'iC'), (3,'iD'), (4,'iE'), (5,'iF')]
)

How would I go about writing definitions such that I could calculate...

a) All ordered subsets.
e.g:
 ['iB']
 ['iA','iD','iE']
 ['iC','iF']
 ['iA','iB','iC','iD','iE','iF']

b) All continuous ordered subset.
e.g:
 ['iB']
 ['iC','iD','iE']
 ['iE','iF']
 ['iA','iB','iC','iD','iE','iF']

c) All fixed relations, such as 2 items spaced by 2.
e.g.
 ['iA','iD']
 ['iC','iF']