sqlalchemy lateral question

import sqlalchemy as sa

#
## setup
#

t1 = sa.table(
    "t1",
    sa.column("id", sa.Integer),
    sa.column("a", sa.Integer),
    sa.column("b", sa.Integer)
)

t2 = sa.table(
    "t2",
    sa.column("id", sa.Integer),
    sa.column("x", sa.Integer),
    sa.column("y", sa.Integer)
)

#
## all auto
#

l1 = sa.select([ (t1.c.a + t2.c.x).label("a_x") ]).lateral("l1")
l2 = sa.select([ (l1.c.a_x + t2.c.y).label("a_x_y") ]).lateral("l2")

join = t1.join(t2, t1.c.id == t2.c.id)\
    .join(l1, sa.true())\
    .join(l2, sa.true())

query = sa.select([ t1.c.a, t2.c.x, l1.c.a_x, l2.c.a_x_y ]).select_from(join)
print(str(query))

"""
sqlalchemy.exc.InvalidRequestError: Select statement 'SELECT t1.a + t2.x AS a_x
FROM t1, t2' returned no FROM clauses due to auto-correlation; specify correlate(<tables>) to control correlation manually.
"""


#
## correlate(None) -> autocorrelation
#

l1 = sa.select([ (t1.c.a + t2.c.x).label("a_x") ]).correlate(None).lateral("l1")
l2 = sa.select([ (l1.c.a_x + t2.c.y).label("a_x_y") ]).correlate(None).lateral("l2")

join = t1.join(t2, t1.c.id == t2.c.id)\
    .join(l1, sa.true())\
    .join(l2, sa.true())

query = sa.select([ t1.c.a, t2.c.x, l1.c.a_x, l2.c.a_x_y ]).select_from(join)

"""
SELECT t1.a, t2.x, l1.a_x, l2.a_x_y
FROM t1 JOIN t2 ON t1.id = t2.id JOIN LATERAL (SELECT t1.a + t2.x AS a_x
FROM t1, t2) AS l1 ON true JOIN LATERAL (SELECT l1.a_x + t2.y AS a_x_y
FROM LATERAL (SELECT t1.a + t2.x AS a_x
FROM t1, t2) AS l1, t2) AS l2 ON true
"""

#
## correlate_except
#

l1 = sa.select([ (t1.c.a + t2.c.x).label("a_x") ]).correlate_except([t1, t2]).lateral("l1")
l2 = sa.select([ (l1.c.a_x + t2.c.y).label("a_x_y") ]).correlate_except([t1, t2]).lateral("l2")

join = t1.join(t2, t1.c.id == t2.c.id)\
    .join(l1, sa.true())\
    .join(l2, sa.true())

query = sa.select([ t1.c.a, t2.c.x, l1.c.a_x, l2.c.a_x_y ]).select_from(join)

"""
sqlalchemy.exc.ArgumentError: FROM expression expected
"""


#
## proof, postgres 10+
#

"""
create table t1 ( id int, a int, b int);
create table t2 (id int, x int, y int);

select t1.a, t2.x, l1.a_x, l2.a_x_y
  from t1
  join t2 on t1.id = t2.id
  join lateral (select t1.a + t2.x as a_x) l1 on true
  join lateral (select l1.a_x + t2.y as a_x_y) l2 on true

"""