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+All thoses modules transforms an expression.
+
+Simple transformations
+----------------------
+
+:Ast:
+
+    Regenerate the Ast from the expression
+
+:Repr:
+
+    Generate a representable string
+
+:Type_of:
+
+    Infer the type of an expression
+
+:Compose:
+
+    Allow to use a module into another one.
+
+:Headers:
+
+    Extract the headers to display in the result file
+
+Composed transformations
+------------------------
+
+:Filter:
+
+    Generate the filters in the query. This module identify if one the
+    expression is actually a group window and handle a special case for this.
+
+    This module relies on Ast
+
+:Query:
+
+    Build an sql query. This module relies on Type_of
+