public interface TextModel
public int length();
public void write(int position, char character);
public char read(int position);
public char text();
Give a pre- and postcondition for read(p) for the
following two situations.
Does the second situation refine the first one?
it returns the character at position p within the text;
it returns the character at position p if p is
smaller than the length of the text and a space otherwise.