Structure Definitionss

Since the posn structure is built into DrScheme, do not try to define it yourself, because it will count as a new kind of posn. Then, your program will not be able to supply posn values to the drawing library, for example, because they will be the wrong kind of posns.

Structure definitions with define-struct are analogous to function definitions, in that the definition can appear either before or after functions using the structure, but must always precede examples using the structure.