key_up
This function checks if a particular key on the keyboard is currently up.
bool key_up(int key)
Parameters:
key
One of the BGT key name constants, see appendix A for a full list.
Return value:
true if the key is up, false if it is not or if an error occurs.
Remarks:
The difference between key_up and key_released is that key_released will only return true when the user first releases the key, while key_up will continue returning true until the key is pushed down again.
Example:

    // Time how long the space bar is up.

    void main()
    {
    timer up;
    show_game_window("keyboard Test");
    up.restart();
    while(!key_pressed(KEY_ESCAPE))
    {
    if(key_up(KEY_SPACE))
    {
    continue;
    }
    alert("Time", "The space bar has been up for "+(up.elapsed/1000)+" seconds.");
    }
    }
    }