Case cade let might supported in 6.5+
Try this pre 6.5
LET fltr = [p.lat, p.lon], lat = [p.lat, p.lon][0], lon = [p.lat, p.lon][1]
LET variable in WHERE clause during plan it will be inline and decide plan.
LET syntax only allows AFTER FROM-JOIN and before WHERE. If never used in WHERE it will process after WHERE (optimization to avoid compute and through away)