These macros define another way to access the structures defined above. For example
The form (fref l [contours @ 2]) gets the third contour of the layer, the @ can be used when the result is a sequence, to refer to a specific element in the sequence.
Setting a value is similar:
The first part between brackets uses the same conventions seen above, the second part is the new value.
The last form fupdate update a value with a procedure.
The effect, here, is shifting a point rightward.