\section{The Theory {\tt odd\_even}}

The theory \ml{odd\_even}\index{odd_even, theory@{\ptt odd\_even}, theory}
contains theorems about the predicates \ml{ODD}\index{ODD@{\ptt
ODD}} and \ml{EVEN}\index{EVEN@{\ptt EVEN}} which test for odd and even
numbers, respectively. 
The theorems that have been pre-proved in this theory include but are not
restricted to the following:

\begin{hol}
\index{EVEN\_ODD\_0@{\ptt EVEN\_ODD\_0}}
\index{EVEN\_ODD\_SUC@{\ptt EVEN\_ODD\_SUC}}
\index{EVEN\_IMPL\_MULT@{\ptt EVEN\_IMPL\_MULT}}
\begin{verbatim}
  EVEN_ODD_0        |- EVEN 0 /\ ~ODD 0

  EVEN_ODD_SUC      |- !n.  (EVEN(SUC n) = ODD n) /\
                            (ODD(SUC n) = EVEN n)

  EVEN_IMPL_MULT    |- !n m. EVEN n \/ EVEN m ==> EVEN(n * m)
\end{verbatim}\end{hol}
