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