use set cardinality instead of counting elements
implements setting an arbitrary element of a list
Sequences emit IndexError, not KeyError
Replace IndexError with KeyError for lists, for better compatibility.  Limited support for list slices.
no, it's IndexError.  (what was i thinking?)
extends on the right end
just one more test
Merge pull request #5 from javierguerragiraldez/master

a little extra lists functionality