Zettel in a [[Sz encoding|00001012920516]] are represented as a [[symbolic expression|00001012930000]].
To process these symbolic expressions, you need to know, how a specific part of a zettel is represented by a symbolic expression.

Basically, each part of a zettel is represented as a list, often a nested list.
The first element of that list is always a unique symbol, which denotes that part.
The meaning / semantic of all other elements depend on that symbol.

=== Zettel
A full zettel is represented by a list of two elements.
The first element represents the metadata, the second element represents the zettel content.

:::syntax
__Zettel__ **=** ''('' [[__Metadata__|#metadata]] [[__Content__|#content]] '')''.
:::

=== Metadata

Metadata is represented by a list, where the first element is the symbol ''META''.
Following elements represent each metadatum[^""Metadatum"" is used as the singular form of metadata.] of a zettel in standard order.

Standard order is: [[Title|00001006020000#title]], [[Role|00001006020000#role]], [[Tags|00001006020000#tags]], [[Syntax|00001006020000#syntax]], all other [[keys|00001006020000]] in alphabetic order.

:::syntax
__Metadata__ **=** ''(META'' [[__Metadatum__|00001012931200]] … '')''.
:::
=== Content

Zettel content is represented by a block.
:::syntax
__Content__ **=** [[__Block__|#block]].
:::

==== Block
A block is represented by a list with the symbol ''BLOCK'' as the first element.
All following elements represent a nested [[block-structured element|00001007030000]].

:::syntax
[!block|__Block__] **=** ''(BLOCK'' [[__BlockElement__|00001012931400]] … '')''.
:::

==== Inline
Both block-structured elements and some metadata values may contain [[inline-structured elements|00001007040000]].
Similar, inline-structured elements are represented as follows:

:::syntax
__Inline__ **=** ''(INLINE'' [[__InlineElement__|00001012931600]] … '')''.
:::

==== Attribute
[[Attributes|00001007050000]] may be specified for both block- and inline- structured elements.
Attributes are represented by the following schema.

:::syntax
__Attribute__ **=** ''('' **[** [[__AttributeKeyValue__|00001012931800]] … **]** ')'.
:::

Either, there are no attributes.
These are specified by the empty list ''()''.
Or there is a list of key / value pairs.

=== Other
==== ''*ZSX-NOT-FOUND:xyz*''
Any symbol with the pattern ''*ZSX-NOT-FOUND:xyz**'', where ""''xyz''"" is any string, signals an internal error.

==== ''*ZSX-SPLICE-NODES*''
A list with ''*ZSX-SPLICE-NODES*'' as its first element may occur during internal processing.
It signals that some processing routine intends to return more than one object instead of just one [[__BlockElement__|00001012931400]] or one [[__InlineElement__|00001012931600]].
The elements of such a list should be ""spliced"" into the parent list.

==== NIL (or ''()'')
Analogous to a splice node, which signals that more than one values is about to be returned, a ""NIL"" value (denoted as ""''()''"") signals that no object should be returned.
Therefore, a [[__BlockElement__|00001012931400]] or an [[__InlineElement__|00001012931600]] may be ""NIL"".

==== ''*ZSX-ID*''
The symbol ''*ZSX-ID*'' is a special [[__Attributes__|00001012931000#attribute]] key.
It denotes an unique identifier.
For example, used for [[heading|00001012931400#heading]] or [[mark|00001012931600#mark]] elements, when needed by encoding the zettel.