key_down
This function checks if a particular key on the keyboard is currently being held down.
bool key_down(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 held down, false if it is not or if an error occurs.
Remarks:
The difference between key_down and key_pressed is that key_pressed will only return true when the user first pushes down the key, while key_down will continue returning true until the key is released again.
Example:

    // Wait for the user to hold down alt and press f4 before closing the program.

    void main()
    {
    show_game_window("Test Game");
    while(true)
    {
    if(key_down(KEY_LMENU) && key_pressed(KEY_F4))
    {
    exit();
    }
    // Other code goes here.
    wait(5);
    }
    }