The interval syntax has the drawback to require an upper value,
which is not true for specs like ">5".

I have implemented a module (attached) which supports:

- Daniel's simple conditions (">a", "<a")
- closed intervals ("[a,b]")
- ISO open intervals ("]a,b[")
- set builder open intervals ("(a,b)")
- boolean operators ("and" and "or", also accepted as "&&" and "||")

The expression is tokenized, and some operators are accepted in variants
known from other programming languages; then, some simple
transformations are performed to create a valid Python expression.

The comma is not accepted as a boolean operator because it is needed
for the interval notion.

I'm not sure whether it does really make sense to support open
intervals; however, it is possible (and done).  Of course, this could be
removed easily.

I'll upload a test module as well; with Python 2.5+, it yields a strange
message:
  Exception exceptions.RuntimeError: 'generator ignored GeneratorExit'
    in <generator object at 0x00AC6E68> ignored
However, all implemented tests succeed.