# The building blocks

Everything in a house is one of four building blocks. This is the house's own account; you
get it in full from `overview` under `model_documentation`.

| Block | What it is |
|---|---|
| **Thing** | a node. Kinds: `Text` (a literal string), `Identity` (an individual, a document, a hierarchy: an identity with no content of its own), `Form` (a record schema defined at runtime), `FormedThing` (a value of a form), `Versioned` (a hull that chains the versions of a slot), `Binary` (bytes identified by their hash), `TimeNode` and `GeoCell` (nodes of the calendar and of the spatial grid) |
| **Link** | an edge that is at the same time a fact. Facts are retracted, never deleted; skills show active facts |
| **Chain** | an ordered list as a value |
| **Tree** | a canonical set as a value; named anchors point at tree roots to form collections such as "Concepts", "Forms" or the instances of an aspect |

## Three things to keep apart

- **Values** are their content. The text "The bridge was finished in 1887." is one node,
  however often you store it; `store_text` answers `is_new: false` the second time. The
  same holds for formed values: equal form and equal values are the same node.
- **Individuals** have an identity of their own. Two calls of `create_person` with the same
  name create two people, on purpose: names are labels, and one name often covers several
  individuals. Creating never searches.
- **Facts** are links. "This claim is stated in that source" is a link you assert and can
  retract. A retracted fact keeps its content, and asserting it again reactivates it.

## IDs

IDs are opaque Base64 strings. Pass them back exactly as received. Every answer names a
node as a triple of `id`, `kind` and `label`, so you can decide what to open next without a
second call.

## The closed vocabulary of standard links

Besides typed links of your own (link forms), there are nine standard meanings, shared by
every house: `Tag`, `Title`, `Index`, `Identify`, `Answer`, `DependOn`, `Promote`,
`Previous`, `PartOf`. The reading rule is always the same: the link is a statement about
its From end, and To is the referent. `PartOf` reads "To contains From"; `Identify` is where
identifiers from foreign systems hang. `assert_link` writes them.

## Time is order, not timestamps

The graph has no modification timestamps. "Current" means position in a version chain, not
a date. When something happened is a fact of its own, asserted with `assert_time`.

## Words you will meet

The skill names and their descriptions are English. Part of the built-in vocabulary is
German, and one German word runs through all descriptions:

| Word | Meaning |
|---|---|
| fassung | a version of a document or of a versioned slot |
| Bestand | a holding: a body of material that belongs together |
| Verdichtung | a digest: a condensed description of a document or a group |
| Gespräch | a conversation |
| Ort, Organisation | place, organisation (built-in aspects beside `Person`) |
| Planung, Vorlage, Agenda | planning, template, agenda (the plan vocabulary) |
| Helfer, Sprecher | helper, speaker |

## Go deeper

- [forms-aspects-concepts](forms-aspects-concepts.md): how to define your own kinds
- [evidence-and-provenance](evidence-and-provenance.md): notes, open questions, what carries a claim
- [finding-things](finding-things.md): the ways to get back what you stored
- [a-memory-you-can-shape](a-memory-you-can-shape.md)