qmk_firmware/keyboards/gboards/g/keymap_combo.h

// Keymap helpers

#define K_ENUM(name, key, ...) name,
#define K_DATA(name, key, ...) const uint16_t PROGMEM cmb_##name[] = {__VA_ARGS__, COMBO_END};
#define K_COMB(name, key, ...) [name] = COMBO(cmb_##name, key),

#define A_ENUM(name, string, ...) name,
#define A_DATA(name, string, ...) const uint16_t PROGMEM cmb_##name[] = {__VA_ARGS__, COMBO_END};
#define A_COMB(name, string, ...) [name] = COMBO_ACTION(cmb_##name),
#define A_ACTI(name, string, ...)         \
    case name:                            \
        if (pressed) SEND_STRING(string); \
        break;

#define A_TOGG(name, layer, ...)          \
    case name:                            \
        if (pressed) layer_invert(layer); \
        break;

#define BLANK(...)
// Generate data needed for combos/actions
// Create Enum
#undef COMB
#undef SUBS
#undef TOGG
#define COMB K_ENUM
#define SUBS A_ENUM
#define TOGG A_ENUM
enum combos {
#include "combos.def"
    COMBO_LENGTH
};
// Export length to combo module
uint16_t COMBO_LEN = COMBO_LENGTH;

// Bake combos into mem
#undef COMB
#undef SUBS
#undef TOGG
#define COMB K_DATA
#define SUBS A_DATA
#define TOGG A_DATA
#include "combos.def"
#undef COMB
#undef SUBS
#undef TOGG

// Fill combo array
#define COMB K_COMB
#define SUBS A_COMB
#define TOGG A_COMB
combo_t key_combos[] = {
#include "combos.def"
};
#undef COMB
#undef SUBS
#undef TOGG

// Fill QMK hook
#define COMB BLANK
#define SUBS A_ACTI
#define TOGG A_TOGG
void process_combo_event(uint16_t combo_index, bool pressed) {
    switch (combo_index) {
#include "combos.def"
    }

    // Allow user overrides per keymap
#if __has_include("inject.h") 
# include "inject.h"
#endif
}
#undef COMB
#undef SUBS
#undef TOGG