text: CREATE OR REPLACE FUNCTION brunnel(is_bridge bool, is_tunnel bool, is_ford bool) RETURNS text AS
  $$
  SELECT CASE
             WHEN is_bridge THEN 'bridge'
             WHEN is_tunnel THEN 'tunnel'
             WHEN is_ford THEN 'ford'
             END;
  $$ LANGUAGE SQL IMMUTABLE
                  STRICT
                  PARALLEL SAFE;
  
  -- The classes for highways are derived from the classes used in ClearTables
  -- https://github.com/ClearTables/ClearTables/blob/master/transportation.lua
  CREATE OR REPLACE FUNCTION highway_class(highway text, public_transport text, construction text) RETURNS text AS
  $$
  SELECT CASE
             %%FIELD_MAPPING: class %%
             END;
  $$ LANGUAGE SQL IMMUTABLE
                  PARALLEL SAFE;
  
  -- The classes for railways are derived from the classes used in ClearTables
  -- https://github.com/ClearTables/ClearTables/blob/master/transportation.lua
  CREATE OR REPLACE FUNCTION railway_class(railway text) RETURNS text AS
  $$
  SELECT CASE
             WHEN railway IN ('rail', 'narrow_gauge', 'preserved', 'funicular') THEN 'rail'
             WHEN railway IN ('subway', 'light_rail', 'monorail', 'tram') THEN 'transit'
             END;
  $$ LANGUAGE SQL IMMUTABLE
                  STRICT
                  PARALLEL SAFE;
  
  -- Limit service to only the most important values to ensure
  -- we always know the values of service
  CREATE OR REPLACE FUNCTION service_value(service text) RETURNS text AS
  $$
  SELECT CASE
             WHEN service IN ('spur', 'yard', 'siding', 'crossover', 'driveway', 'alley', 'parking_aisle') THEN service
             END;
  $$ LANGUAGE SQL IMMUTABLE
                  STRICT
                  PARALLEL SAFE;
  
  -- Limit surface to only the most important values to ensure
  -- we always know the values of surface
  CREATE OR REPLACE FUNCTION surface_value(surface text) RETURNS text AS
  $$
  SELECT CASE
             WHEN surface IN ('paved', 'asphalt', 'cobblestone', 'concrete', 'concrete:lanes', 'concrete:plates', 'metal',
                              'paving_stones', 'sett', 'unhewn_cobblestone', 'wood') THEN 'paved'
             WHEN surface IN ('unpaved', 'compacted', 'dirt', 'earth', 'fine_gravel', 'grass', 'grass_paver', 'gravel',
                              'gravel_turf', 'ground', 'ice', 'mud', 'pebblestone', 'salt', 'sand', 'snow', 'woodchips')
                 THEN 'unpaved'
             END;
  $$ LANGUAGE SQL IMMUTABLE
                  STRICT
                  PARALLEL SAFE;
  
  -- Determine which transportation features are shown at zoom 12
  CREATE OR REPLACE FUNCTION transportation_filter_z12(highway text, construction text) RETURNS boolean AS
  $$
  SELECT CASE
             WHEN highway IN ('unclassified', 'residential') THEN TRUE
             WHEN highway_class(highway, '', construction) IN
                 (
                  'motorway', 'trunk', 'primary', 'secondary', 'tertiary', 'raceway',
                  'motorway_construction', 'trunk_construction', 'primary_construction',
                  'secondary_construction', 'tertiary_construction', 'raceway_construction'
                 ) THEN TRUE --includes ramps
             ELSE FALSE
         END
  $$ LANGUAGE SQL IMMUTABLE
                  STRICT
                  PARALLEL SAFE;
  
  -- Determine which transportation features are shown at zoom 13
  -- Assumes that piers have already been excluded
  CREATE OR REPLACE FUNCTION transportation_filter_z13(highway text,
                                                       public_transport text,
                                                       construction text,
                                                       service text) RETURNS boolean AS
  $$
  SELECT CASE
             WHEN transportation_filter_z12(highway, construction) THEN TRUE
             WHEN highway = 'service' OR construction = 'service' THEN service NOT IN ('driveway', 'parking_aisle')
             WHEN highway_class(highway, public_transport, construction) IN ('minor', 'minor_construction') THEN TRUE
             ELSE FALSE
         END
  $$ LANGUAGE SQL IMMUTABLE
                  STRICT
                  PARALLEL SAFE;
  