Children with expressive language disorder have difficulty conveying or expressing information in speech, writing, sign language or gesture. (For preschool children, the difficulty expressing themselves in writing is not evident, as they have not started formal education.)
A rectangle is defined as a parallelogram (a quadrilateral with two sets of parallel sides) with the non-parallel sides joining perpendicularly (at 90Â°). A square is a rectangle with the additional constraint the all four sides be of equal length.
As such we can say that a square is a  type of rectangle which is a type of shape. We can therefore pass a  square into any function that expects a shape or rectangle object  because a square is covariant with both a rectangle and a shape.