We need a spec

Currently, the Handlebars language is defined by the behavior of this reference implementation and incompletely described by the documentation found at handlebarsjs.com. This makes it very difficult to reason about bugs like #1276. Should partials that use `{{> @partial-block}}` be invocable in the manner the bug reporter expects? Not clear.

I had a related discussion with @wycats and we came to the conclusion that we need to specify the language before we move forward with any new features.

This would be a non-trivial effort. Thoughts?

Start by reviewing the existing documentation at handlebarsjs.com, the behavior of the reference implementation, and bug #1276, especially the question about partials using `{{> @partial-block}}`. A complete result would define the Handlebars language sufficiently to resolve such semantic questions before new features are added.

Written by the indexing model from the issue text.