Ladder or Stepladder?

[Mercen-X]

A square is a rectangle, but not all rectangles are squares. Some are, though.
A stepladder is a ladder, but not all ladders are stepladders. Some are, though.

All step-parents are parents, but not all parents are step-parents. Some are, though.
All step-children are children, but not all children are step-children. Some are, though.
All rectangles are polygons, but not all polygons are rectangles. Some are, though.
All prime numbers are real numbers, but not all real number are prime numbers. Some are, though.
All dvds are disks, but not all disks are dvds. Some are, though.

There is no end to this.

EDIT:

OOC:	Also, my ladder left me. Now I have a stepladder. I miss my ladder.