Forms of the language

The reserved heads the checker matches on — and therefore the names nothing may be called.

Doc comments

## in the Python surface and ;; in the S-expression one, on the lines immediately above a declaration. A blank line ends a run. beck fmt preserves them; an ordinary # comment it does not, because the lexer discards it.

## One item on the list.
model Todo:
    ## Stable for the life of the item.
    id: Id

A doc comment is metadata, not a form: documenting a definition does not change the program's meaning and does not move the module's interface digest.