Update documentation for DECONSTRUCT

Add documentation describing how the DECONSTRUCT statement can be used.

docs/bql.md

@@ -428,7 +428,7 @@ The where clause will bind to all available predicates. Each binding will
 then be used in the construct part to create the new triples.
 
 A more elaborate example would be to create a new fact `grandparent` into
-the destination graphs by properly extracting graph patterns via the where
+the destination graphs by properly extracting graph patterns via the `WHERE`
 clause.
 
 ```
@@ -517,3 +517,51 @@ number of blank nodes. The syntax is always the same, they all start with
 the prefix `_:` followed by a logical ID. On insertion of each new fact,
 BQL guarantees a new unique blank node will be generated by each of them.
 Example of multiple blank nodes generated at once are `_:v0`, `_:v1`, etc.
+
+
+## Removing complex facts out of existing graphs using existing statements
+
+In some cases you want to create remove facts--remove existing triples---in an
+existing graph or graphs based on facts existing on the same or other graphs. 
+Deconstructing facts requires two steps: (1) querying for the information that
+is going to be used to remove facts, and (2) how the facts to be removed are
+going to be assembled. An illustrative example is:
+
+```
+  DECONSTRUCT {
+    ?s ?p ?o
+  }
+  IN ?dest
+  FROM ?src
+  WHERE {
+    ?s ?p ?o
+  };
+```
+
+The above example would remove all immutable triples in the `?dest` graph that
+were found in the `?src` graph. As the `CONSTRUCT` statement, the `DECONTRUCT`
+statement supports multiple graph on the `IN` and `FROM` clauses of the
+statement.
+
+Following the example used in the previous section, we could use the
+`DECONSTRUCT` to remove `grandparent` facts in a destination graph built from
+a source graphs by properly extracting graph patterns via the `WHERE`
+clause.
+
+```
+  DECONSTRUCT {
+    ?ancestor "grandparent"@[] ?grandchildren
+  }
+  INTO ?dest
+  FROM ?src
+  WHERE {
+    ?ancestor "parent"@[] ?c .
+    ?c "parent"@[] ?grandchildren
+  };
+```
+
+The `DECONSTRUCT` statement does not support neither the blank node notation
+nor the reification syntax. Those are used to refer to newly created nodes
+introduced by the statement, which makes sends on `CONSTRUCT` statements.
+However, `DECONSTRUTC` statements already have all the required information
+to assemble the triples to remove.