SPECIFICATION

You are asked to write a Scheme procedure, called cover, that covers an $n\times n$chessboard with k knights. More precisely,

   (cover n k)

should yield a sentence with k words each showing the position of a knight on the board. If a coverage is not possible the yielded sentence should be empty.

A word will be of the form cr where c is a letter a through i denoting a column and r is a digit 1 through 9 denoting a row where a1 denotes the southwest corner. This is a standard notation for positions on a chess board.

Notice we have assumed that

$$1\leq n \leq 9$$

to simplify data representation.