Improve Problem description for Rectangles

The Rectangles problem asks users to count the number of rectangles in an array of ASCII text. In the problem description, an illustration of such an array is given, and its valid rectangles are called out.

However, there's a sentence at the end of the problem description that seems unhelpful:

> You may assume that the input is always a proper rectangle (i.e. the length of every line equals the length of the first line).

What does this mean? It's confusing because:
1. The sides of the rectangles do not need to all be equal (that's a square of course, which is a special case of a rectangle.)
2. The "first line" means what?

I would propose that this sentence simply be removed, since knowing what a rectangle is seems pretty fundamental, and this sentence doesn't make anything clearer. If for some reason there needs to be explanatory text there, I think it should be helpful and unambiguous, perhaps like:

> A rectangle's opposite sides will be both parallel and equal to each other in length. A rectangle's adjacent sides may or may not be equal to each other in length. You can see in the above examples that some rectangles have a height that is equal to their width, and some do not.

Or maybe the sentence is intended to convey something different than what a rectangle is? If that's the case, then I still think it could be clarified to communicate whatever is actually intended.