Query DSL - ECS_SYSTEM : [out] pair in empty entity with Sync Point

[Felecarpp]

Hello, I'm new to C and flecs. I would like to create a sync point between two systems using a relationship tag. I succeeded with the C binding for query building but I failed with the DSL.
Here is a restricted example : the first system create a new pair (DockedTo, earth) and the second system query (DockedTo, *). The goal is to have a sync point so the both systems must write a message in sdtout in the same round.

#include <stdio.h>
#include "flecs.h"

typedef struct { int instant, maximum; } Condition;

ECS_TAG_DECLARE(DockTo);
ECS_TAG_DECLARE(DockedTo);
ECS_COMPONENT_DECLARE(Condition);

/* Dock a Docking spaceship to its target */
void Docking(ecs_iter_t *it) {
    ecs_world_t *ecs = it->world;
    ecs_entity_t target = ecs_pair_second(ecs, ecs_field_id(it, 1));

    for (int i = 0; i < it->count; i++) {
        ecs_entity_t spaceship = it->entities[i];
        ecs_remove_pair(ecs, spaceship, DockTo, target);
        ecs_add_pair(ecs, spaceship, DockedTo, target);
        printf("spaceship dock to %lu\n", target);
    }
}

/* Repair a Docked spaceship */
void Repair(ecs_iter_t *it) {
    Condition *p = ecs_field(it, Condition, 1);

    for (int i = 0; i < it->count; i++) {
        if(p[i].instant < p[i].maximum)
            p[i].instant += 2;
        if(p[i].instant > p[i].maximum)
            p[i].instant = p[i].maximum;
        printf("spaceship repaired to %d/%d\n", p[i].instant, p[i].maximum);
    }
}


int main(int argc, char *argv[]) {
    ecs_world_t *ecs = ecs_init_w_args(argc, argv);

    ECS_TAG_DEFINE(ecs, DockTo);
    ECS_TAG_DEFINE(ecs, DockedTo);
    ECS_COMPONENT_DEFINE(ecs, Condition);

    ecs_system(ecs, {
        .entity = ecs_entity(ecs, {
             .name = "Docking",
             .add = { ecs_dependson(EcsOnUpdate) },
         }),
         .query.filter.terms = {
             {
                 .inout = EcsIn,
                 .id = ecs_pair(DockTo, EcsWildcard),
             },
             {
                 .inout = EcsOut,
                 .id = ecs_pair(DockedTo, EcsWildcard),
                 .src.flags = EcsIsEntity,
                 .src.id = 0,
             },
         },
         .callback = Docking,
    });
    ECS_SYSTEM(ecs, Repair, EcsOnUpdate, [in] Condition, [in] (DockedTo, *));

    ecs_entity_t spaceship = ecs_new_id(ecs);
    ecs_entity_t earth = ecs_new_id(ecs);

    ecs_set(ecs, spaceship, Condition, {4, 16});
    ecs_add_pair(ecs, spaceship, DockTo, earth);
    
    ecs_progress(ecs, 1);
    
    return ecs_fini(ecs);
}

$ cc main.c flecs.c -o spaceship
$ ./spaceship
spaceship dock to 533
spaceship repaired to 6/16

This works well. But I'd like to do the same with the DSL to save a few lines in the source code. I replaced the ecs_system call by ECS_SYSTEM(ecs, Docking, EcsOnUpdate, [in] (DockTo, *), [out] (DockedTo, *)); but I get an error :

error: Docking: expected end of expression or next term
[in] (DockTo, *), [out] (DockedTo, *)()
                                     ^
fatal: flect_test.c: 44: assert: ecs_id(Docking) != 0 INVALID_PARAMETER             
./spaceship(+0x3b86b) [0x5579fe2ff86b]                                                                                                   
./spaceship(+0x62602) [0x5579fe326602]
./spaceship(+0x13377) [0x5579fe2d7377]
./spaceship(+0xaf33) [0x5579fe2cef33]                                                                                                    
./spaceship(+0x2b14) [0x5579fe2c6b14]
/usr/lib/libc.so.6(+0x27cd0) [0x7ff7c48d7cd0]
/usr/lib/libc.so.6(__libc_start_main+0x8a) [0x7ff7c48d7d8a]
./spaceship(+0x2505) [0x5579fe2c6505]
Abandon (core dumped)

I conclude this syntax do not exist. But just I mixed a syntax from the documentation with relationships so I do not understand why.
https://www.flecs.dev/flecs/md_docs_Systems.html#autotoc_md290

[ECS_SYSTEM](world, SetTransform, EcsOnUpdate, Position, [out] Transform());

Is there a DSL syntax to do this ?

[SanderMertens]

The correct DSL syntax for this is:

[out] DockedTo(0, *)

The reason for this is that (Foo, Bar) is short for Foo($this, Bar), where $this is a builtin variable that represents the entity on which components got matched. Replacing that with 0 is equivalent to what happens in the C API.