+** structures
+ structures group things together, like lists. structures group a
+ fixed number of items, unlike lists.
+ example: student has name ID dorm.
+*** structure basics
+ * "(struct student (name id# dorm))"
+ this defines a structure, but doesn't actually create one.
+ * "(define freshman1 (student 'Joe 1234 'NewHall))"
+ this creates a structure
+ * fields: name, id#, dorm
+ * (equal? (student-name freshman1) 'Joe)