[scrumper]

Don't think of it as an API. It's a complete and very rich language that supports functional and procedural (and declarative, and other) programming models.

To your question, it's not so much that V6 is opening documents written in V1, it's more that V6 of the language runs V1 code. Viewed through that lens, it's not such a surprise. K&R C still compiles, after all. (In fact, certain peripheral pieces of older code won't actually work in newer versions. It's not uncommon for old notebooks to require some changes.)

The fundamental heart of Mathematica and its language is quite unusual: it is a Term Rewriting System (TRS.) There's a good set of examples here [1] but basically a TRS works by the repeated application of matching patterns to expression terms and modifying them with replacement rules. The language's flexibility comes from the clever use of those rules.

You can do amazing things with it very easily if you follow its way. It's pretty unique in the world of programming languages and a 30 day trial will be fun for anyone interested in experimenting with languages.

[1] http://www.cs.swan.ac.uk/~csneal/SystemSpec/termrewriting.ht...