registering a function for callback is not working

[ripred]

/*\ Example use of the ButtonGestures library
|*|
|*| This example shows how to register different functions to be
|*| called for different gestures.
|*|
|*| If a function has been registered to a gesture then the function
|*| will automatically be called for that gesture.
\*/

#include <ButtonGestures.h>

// NOTE: change/define the following pin(s) based on your project/connections
#define   BUTTON_PIN    2

ButtonGestures  button(BUTTON_PIN);

//
// These functions will be registered and called for the various gestures:
//

void function1(const uint8_t /*pin*/, const uint8_t /*state*/) {
    Serial.println("function 1 has been called");
}

void function2(const uint8_t /*pin*/, const uint8_t /*state*/) {
    Serial.println("function 2 has been called");
}

void function3(const uint8_t /*pin*/, const uint8_t /*state*/) {
    Serial.println("function 3 has been called");
}

void function4(const uint8_t /*pin*/, const uint8_t /*state*/) {
    Serial.println("function 4 has been called");
}

void function5(const uint8_t /*pin*/, const uint8_t /*state*/) {
    Serial.println("function 5 has been called");
}

void function6(const uint8_t /*pin*/, const uint8_t /*state*/) {
    Serial.println("function 6 has been called");
}


void setup(void) {
    Serial.begin(2000000);
    uint32_t timer = millis() + 2000;
    while (!Serial && millis() < timer);
    Serial.flush();
    Serial.println(F("\n\nArduino Core Library - ButtonGestures Library Test"));

    if (!button.set_callback(SHORT1, function1))
        Serial.println("Error registering callback for SHORT1");
    if (!button.set_callback(LONG1, function2))
        Serial.println("Error registering callback for LONG1");
    if (!button.set_callback(SHORT2, function3))
        Serial.println("Error registering callback for SHORT2");
    if (!button.set_callback(LONG2, function4))
        Serial.println("Error registering callback for LONG2");
    if (!button.set_callback(SHORT3, function5))
        Serial.println("Error registering callback for SHORT3");
    if (!button.set_callback(LONG3, function6))
        Serial.println("Error registering callback for LONG3");
}

void loop(void) {
    button.check_button();
}

This example should display the appropriate debug output when each registered function is invoked by making the associated gesture with the push button.

[ripred]

fixed in v2.0.1