Comments Anything that is not formal. If D is a collection of declarations, definitions, axioms, theorems etc. Each course is offered by one dept. Each qualification belongs to a department. Each section covers one course.
Students are people.
A proposition can have two possible meanings - true and false. The syntax attempts to make the writing of formal statements as natural as possible. It is therefore unambiguous to use sets to declare the type of a variable. In mathematics '.' projects a vector into a scalar. The formal meaning of a structure is as a collection of maps.