What are the preconditions and postconditions for this function?

A function foo is developed to determine the shape of a polygon with four sides a, b, c, and d. The function also accepts a degree of angle e in between the two sides a and b as the fifth argument. The function foo is expected to be able to determine these five different types of quadrilaterals: parallelogram, rectangle, rhombus, square, and kite. If a shape is not one of these five types, it is considered as irregular despite the shape may be a trapezoid.