How to use the geoalchemy2.functions.GenericFunction function in GeoAlchemy2

class ST_Multi(GenericFunction):
    name = 'ST_Multi'
    type = Geometry


class ST_Collect(GenericFunction):
    name = 'ST_Collect'
    type = Geometry


class ST_Union(GenericFunction):
    name = 'ST_Union'
    type = Geometry


class ST_Buffer(GenericFunction):
    name = 'ST_Buffer'
    type = Geometry


class ST_SetSRID(GenericFunction):
    name = 'ST_SetSRID'
    type = Geometry


DBSession = scoped_session(sessionmaker(extension=ZopeTransactionExtension()))
Base = declarative_base()
make_translatable()

users_licenses_table = Table(
    'users_licenses', Base.metadata,
    Column('user', BigInteger, ForeignKey('users.id')),

class ST_Buffer(GenericFunction):
    """ Exposes PostGIS ST_Buffer function """

    name = "ST_Buffer"
    type = Geometry


class ST_Intersects(GenericFunction):
    """ Exposes PostGIS ST_Intersects function """

    name = "ST_Intersects"
    type = Geometry


class ST_MakeEnvelope(GenericFunction):
    """ Exposes PostGIS ST_MakeEnvelope function """

    name = "ST_MakeEnvelope"
    type = Geometry


class ST_X(GenericFunction):
    """ Exposes PostGIS ST_X function """

    name = "ST_X"
    type = Geometry


class ST_Y(GenericFunction):
    """ Exposes PostGIS ST_Y function """

)

from pyramid.threadlocal import get_current_registry


class ST_Multi(GenericFunction):
    name = 'ST_Multi'
    type = Geometry


class ST_Collect(GenericFunction):
    name = 'ST_Collect'
    type = Geometry


class ST_Union(GenericFunction):
    name = 'ST_Union'
    type = Geometry


class ST_Buffer(GenericFunction):
    name = 'ST_Buffer'
    type = Geometry


class ST_SetSRID(GenericFunction):
    name = 'ST_SetSRID'
    type = Geometry


DBSession = scoped_session(sessionmaker(extension=ZopeTransactionExtension()))
Base = declarative_base()

class ST_Collect(GenericFunction):
    name = 'ST_Collect'
    type = Geometry


class ST_Union(GenericFunction):
    name = 'ST_Union'
    type = Geometry


class ST_Buffer(GenericFunction):
    name = 'ST_Buffer'
    type = Geometry


class ST_SetSRID(GenericFunction):
    name = 'ST_SetSRID'
    type = Geometry


DBSession = scoped_session(sessionmaker(extension=ZopeTransactionExtension()))
Base = declarative_base()
make_translatable()

users_licenses_table = Table(
    'users_licenses', Base.metadata,
    Column('user', BigInteger, ForeignKey('users.id')),
    Column('license', Integer, ForeignKey('licenses.id')))

# user roles
ADMIN = 1
PROJECT_MANAGER = 2

class ST_Centroid(GenericFunction):
    """ Exposes PostGIS ST_Centroid function """

    name = "ST_Centroid"
    type = Geometry


class ST_Transform(GenericFunction):
    """ Exposes PostGIS ST_Transform function """

    name = "ST_Transform"
    type = Geometry


class ST_Area(GenericFunction):
    """ Exposes PostGIS ST_Area function """

    name = "ST_Area"
    type = None


class ST_Buffer(GenericFunction):
    """ Exposes PostGIS ST_Buffer function """

    name = "ST_Buffer"
    type = Geometry


class ST_Intersects(GenericFunction):
    """ Exposes PostGIS ST_Intersects function """

docs.append(doc[0])
        docs.append('see http://postgis.net/docs/{0}.html'.format(doc[1]))
    elif doc is not None:
        docs.append(doc)
        docs.append('see http://postgis.net/docs/{0}.html'.format(name))

    if type_ is not None:
        attributes['type'] = type_

        type_str = '{0}.{1}'.format(type_.__module__, type_.__name__)
        docs.append('Return type: :class:`{0}`.'.format(type_str))

    if len(docs) != 0:
        attributes['__doc__'] = '\n\n'.join(docs)

    globals()[name] = type(name, (GenericFunction,), attributes)


#
# Define compiled versions for functions in SpatiaLite whose names don't have
# the ST_ prefix.
#


_SQLITE_FUNCTIONS = {
    "ST_GeomFromEWKT": "GeomFromEWKT",
    "ST_GeomFromEWKB": "GeomFromEWKB",
    "ST_AsBinary": "AsBinary",
    "ST_AsEWKB": "AsEWKB",
    "ST_AsGeoJSON": "AsGeoJSON",
}

return datetime.datetime.utcnow()


class ST_GeomFromText(GenericFunction):
    """ Export the postgis ST_GeomFromText function """
    name = 'ST_GeomFromText'
    type = Geometry


class ST_Intersects(GenericFunction):
    """ Exposes PostGIS ST_Intersects function """
    name = 'ST_Intersects'
    type = Geometry


class ST_MakeEnvelope(GenericFunction):
    """ Exposes PostGIS ST_MakeEnvelope function """
    name = 'ST_MakeEnvelope'
    type = Geometry


class ST_AsText(GenericFunction):
    """ Exposes PostGIS ST_AsText function """
    name = 'ST_AsText'
    type = Geometry

class ST_Area(GenericFunction):
    """ Exposes PostGIS ST_Area function """

    name = "ST_Area"
    type = None


class ST_Buffer(GenericFunction):
    """ Exposes PostGIS ST_Buffer function """

    name = "ST_Buffer"
    type = Geometry


class ST_Intersects(GenericFunction):
    """ Exposes PostGIS ST_Intersects function """

    name = "ST_Intersects"
    type = Geometry


class ST_MakeEnvelope(GenericFunction):
    """ Exposes PostGIS ST_MakeEnvelope function """

    name = "ST_MakeEnvelope"
    type = Geometry


class ST_X(GenericFunction):
    """ Exposes PostGIS ST_X function """

from sqlalchemy_i18n import (
    Translatable,
    make_translatable,
    translation_base,
)

from pyramid.threadlocal import get_current_registry


class ST_Multi(GenericFunction):
    name = 'ST_Multi'
    type = Geometry


class ST_Collect(GenericFunction):
    name = 'ST_Collect'
    type = Geometry


class ST_Union(GenericFunction):
    name = 'ST_Union'
    type = Geometry


class ST_Buffer(GenericFunction):
    name = 'ST_Buffer'
    type = Geometry


class ST_SetSRID(GenericFunction):
    name = 'ST_SetSRID'

class VersionNotFound(Exception):
    """ Custom exception to indicate that model version is not found """
    pass


class PredictionsNotFound(Exception):
    """ Custom exception to indicate that no predictions were found """


def timestamp():
    """ Used in SQL Alchemy models to ensure we refresh
    timestamp when new models initialised"""
    return datetime.datetime.utcnow()


class ST_GeomFromText(GenericFunction):
    """ Export the postgis ST_GeomFromText function """
    name = 'ST_GeomFromText'
    type = Geometry


class ST_Intersects(GenericFunction):
    """ Exposes PostGIS ST_Intersects function """
    name = 'ST_Intersects'
    type = Geometry


class ST_MakeEnvelope(GenericFunction):
    """ Exposes PostGIS ST_MakeEnvelope function """
    name = 'ST_MakeEnvelope'
    type = Geometry