Often, I know that a variable should have one of a set of values, and I want to determine which one, with an if/elif/else clause. This looks something like this:
```
if foo == 1:
    # do a
elif foo == 2:
    # do b
elif foo == 3:
    # do c
else:
    raise ValueError('foo must be 1, 2 or 3')
```
Sometimes, I just do
```
if foo == 1:
    # do a
elif foo == 2:
    # do b
else:
    # do c
```
But this is less readable and allows errors to slip past. My proposal is to allow the following syntax:
```
if foo == 1:
    # do a
elif foo == 2:
    # do b
else foo == 3:
    # do c
```
Or perhaps the more readable version:
```
if foo == 1:
    # do a
elif foo == 2:
    # do b
else assert foo == 3:
    # do c
```
Both of these options are backward compatible, since currently nothing is allowed between the `else` and the `:`.
This would be roughly equivalent to
```
if foo == 1:
    # do a
elif foo == 2:
    # do b
else:
    assert foo == 3
    # do c
```
But shorter and more readable, since it puts the assertion at the same levels as the others. Thoughts?
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