i'm lost in list manipulation

[GrelEns]

hello,

having a list like ['a', 'b', 'c', 'd'] i would like to get

[['a', '-', 'b', 'c', 'd'],
['a', 'b', '-', 'c', 'd'],
['a', 'b', 'c', '-', 'd'],
['a', '-', 'b', '-', 'c', 'd'],
['a', 'b', '-', 'c', '-', 'd'],
['a', '-', 'b', '-', 'c','-', 'd']]

that is adding successively an item between at all the available place, and
i just can't get my brain doing that :(

thank a lot if you could help...