How does Gram notation relate to Pattern?

Gram notation is a serialisation of Pattern<Subject>, not a separate data model. There is no "Gram data structure" — when you parse Gram, you get back Pattern<Subject> values. When you stringify patterns, you get back Gram text.

The relationship is:

Gram string  →  parse()   →  [Pattern<Subject>, ...]
[Pattern<Subject>, ...]  →  stringify()  →  Gram string

A valid Pattern<Subject> always round-trips: stringify(parse(s)) == s (up to normalisation of whitespace and property ordering).

This means Gram is not a layer on top of Pattern — it is Pattern, expressed as text. The Gram codec (the gram-codec crate / relateby.gram module / @relateby/gram package) provides the translation in both directions, but the underlying type is always Pattern<Subject>.

Gram notation can express any Pattern<Subject>. The shorthand forms (parentheses, arrows) are editorial sugar for common shapes. The general [value | elem, elem] form covers everything else.