This tutorial example is taken out of a VDM course given
to the students of the Dipl\^{o}me d'Etudes Sup\'erieures Sp\'ecialis\'ees en
G\'enie Informatique (5th year) at the Universit\'e Joseph Fourier.
This
example uses the implicit style of specification of VDM-SL and thus may not be
executed with the IFAD toolbox.
\section{Informal statement}
The crosswords assistant is a simple system which helps writing crosswords.
Its user places words or black squares on a a crossword grid.
The system helps him keep a log of the words that appear on the grid.
These words appear in a list of words (waiting list).
The user will then informally check that these words effectively exist. Once a
word has been checked by the user, it will be validated and stored in a second
list.
\subsection*{Example}
The user has placed sucessively the words {\tt word}, {\tt coal} and {\tt art}
on the grid.
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