# Forth

Implement an evaluator for a very simple subset of Forth.

[Forth](https://en.wikipedia.org/wiki/Forth_%28programming_language%29)
is a stack-based programming language. Implement a very basic evaluator
for a small subset of Forth.

Your evaluator has to support the following words:

- `+`, `-`, `*`, `/` (integer arithmetic)
- `DUP`, `DROP`, `SWAP`, `OVER` (stack manipulation)

Your evaluator also has to support defining new words using the
customary syntax: `: word-name definition ;`.

To keep things simple the only data type you need to support is signed
integers of at least 16 bits size.

You should use the following rules for the syntax: a number is a
sequence of one or more (ASCII) digits, a word is a sequence of one or
more letters, digits, symbols or punctuation that is not a number.
(Forth probably uses slightly different rules, but this is close
enough.)

Words are case-insensitive.

## Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to
indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not
every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include
a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of
`raise Exception`, you shold write:

```python
raise Exception("Meaningful message indicating the source of the error")
```

## Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the `$EXERCISM_WORKSPACE/python/forth` directory.

You can find your Exercism workspace by running `exercism debug` and looking for the line that starts with `Workspace`.

For more detailed information about running tests, code style and linting,
please see the [help page](http://exercism.io/languages/python).

## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.