\section*{Definitions}
\label{definitions}

(1) We define here a set of usual terms in this paper.\\

\begin{mydef} Valid identifier.
\\An identifier is valid if it is composed only by the letters 
of the alphabet (upper-case or lower-case), digits from 0 to 9 
and characters $.$, $:$, $/$, $\backslash$ and $_{-}$.
\end{mydef}