Merge pull request #34 from zvr/reword-id-structure

Reword the SWHID structure description

Chapters/5.Core_identifiers.md

@@ -1,22 +1,27 @@
 # 5 Core identifiers
 
-A core SWHID identifier is composed of four fields, separated by a colon
-`:`.  The `swh` prefix makes it explicit that this is a SWHID identifier.  `1`
-(`<scheme_version>`) is the current version of this identifier *scheme*. Future
-editions will use higher version numbers, possibly breaking backward
-compatibility. The third field is a tag that correspond to the type of object
+A core SWHID identifier is composed of four fields, separated by a colon `:`.
+
+The first field is the *type of the identifier*
+and it is defined to be `swh`.
+
+The second field is the *version of the identifier scheme*
+and for this version of the specification
+it is defined to be `1`.
+
+The third field is a tag corresponding to the *type of object*
 identified:
 
-- `cnt` for **contents**.
-- `dir` for **directories**,
-- `rev` for **revisions**,
-- `rel` for **releases**,
-- `snp` for **snapshots**,
+- `cnt` for **contents** (see 5.1)
+- `dir` for **directories** (see 5.2)
+- `rev` for **revisions** (see 5.3)
+- `rel` for **releases** (see 5.4)
+- `snp` for **snapshots** (see 5.5)
+
+The fourth field is the *intrinsic identifier* of the object.
+This is a hex-encoded (using lowercase ASCII characters) hash value
+computed by the content and relevant metadata of the object.
 
-The fourth and last field is the *intrinsic identifier* of the object.  In this
-version of the specification, this is a hex-encoded (using lowercase ASCII
-characters) SHA1 computed on the content and relevant metadata of the object
-itself, as follows.
 ## 5.1 Contents
 
 A *content* is an uninterpreted byte sequence, typically, the content of a file.