yprevious
Returns: Real
It can often be useful to know where an instance was the
previous step, when resolving collisions for example, and so you
can use this function to get that value for use. It is normally
stored just before the start of the begin step event but can
be set through code at any time, meaning you can give it your own
custom value should that be necessary.
if x != xprevious || y != yprevious { moved = true; }
The above code checks to see if there is any difference between the yprevious and yprevious values and the current x and y values. If there is it sets a variable to true.