More docs.

doc/sql.extensions/README.distinct

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+------------------
+DISTINCT predicate
+------------------
+
+  Function:
+    Specify a test of whether two row values are distinct.
+
+  Author:
+    Oleg Loa <loa@mail.ru>
+    Dmitry Yemanov <yemanov@yandex.ru>
+
+  Syntax rules:
+    <value> IS [NOT] DISTINCT FROM <value>
+
+  Scope:
+    DSQL, PSQL
+
+  Example(s):
+    1. SELECT * FROM T1 JOIN T2 ON T1.NAME IS NOT DISTINCT FROM T2.NAME;
+    2. SELECT * FROM T WHERE T.MARK IS DISTINCT FROM 'test'
+
+  Note(s):
+    1. A DISTINCT predicate evaluates very similar to an equality predicate with
+       the only difference that two NULL values are considered not distinct. As a result,
+       this predicate never evaluates to UNKNOWN truth value (the same as IS [NOT] NULL
+       predicate behaves).
+    2. The NOT DISTINCT predicate can be optimized via an index, if exists.

doc/sql.extensions/README.sequence_generators

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+-------------------
+Sequence generators
+-------------------
+
+  Function:
+    A sequence generator is a mechanism for generating successive exact
+    numeric values, one at a time. A sequence generator is a named schema
+    object.
+
+  Syntax rules:
+    CREATE { SEQUENCE | GENERATOR } <name>
+    DROP { SEQUENCE | GENERATOR } <name>
+    SET GENERATOR <name> TO <start_value>
+    ALTER SEQUENCE RESTART WITH <start_value>
+    GEN_ID (<name>, <increment_value>)
+    NEXT VALUE FOR <name>
+
+  Type:
+    INTEGER in dialect 1, BIGINT in dialect 3
+
+  Example(s):
+    1. CREATE SEQUENCE S_EMPLOYEE;
+    2. ALTER SEQUENCE S_EMPLOYEE RESTART WITH 0;
+    3. SELECT GEN_ID(S_EMPLOYEE, 1) FROM RDB$DATABASE;
+    4. INSERT INTO EMPLOYEE (ID, NAME) VALUES (NEXT VALUE FOR S_EMPLOYEE, 'John Smith');
+
+  Note(s):
+    1. SEQUENCE is a syntax term declared in the SQL specification, while
+       GENERATOR is a legacy InterBase syntax term. It's recommended to use
+       the standard SEQUENCE syntax in your applications.
+    2. Currently, increment values not equal to 1 (one) could be used only via
+       the GEN_ID function. The future versions are expected to provide full
+       support for SQL-99 sequence generators (which allows the required
+       increment values to be specified at the DDL level), so it's recommended
+       to get new sequential values via NEXT VALUE FOR value expression instead
+       of the GEN_ID function, unless it's vitally important to use other
+       increment values.
+    3. GEN_ID(<name>, 0) allows you to retrieve the current sequence value,
+       but it should be never used in insert/update statements, as it produces a
+       high risk of uniqueness violations in a concurrent environment.