Fact  {
    0 = 1;
    sN = <Int 'N:='('"'sN)';'
              Prod ('I:=1,N') ('I')
         >;
}

Int { eX = <Eval () eX>; }

Eval {
   (eE) sV ':='tX';'eY            = <Eval (eE (sV <Eval (eE) tX>)) eY>;
    tE  sN                        = <Subst tE sN>;
    tE  ('"'eX)                   = eX;
    tE  (eX)                      = <Eval tE eX>;
    tE  eX '=' eY                 = <Equ (<Eval tE eX>) <Eval tE eY>>;
    tE  eX '+' eY                 = <ADD (<Eval tE eX>) <Eval tE eY>>;
    tE  eX '-' eY '-' eZ          = <Eval tE (eX '-' eY) '-' eZ>;
    tE  eX '-' eY                 = <SUB (<Eval tE eX>) <Eval tE eY>>;
    tE  eX '*' eY                 = <MUL (<Eval tE eX>) <Eval tE eY>>;
    tE  sF (eX)                   = <Func sF <Eval tE eX>>;
    tE  If tC Then tT Else tF     = <Eval tE <If <Eval tE tC> tT tF>>;
    tE  Prod (sV':='eL','eH) (eX) = <Prod tE sV (<Eval tE eL>) (<Eval tE eH>) (eX)>;
}

Prod {
    tE sV (sL) (sL) (eX) =       <Eval tE sV':='('"'sL)';'eX>;
    tE sV (sL) (sH) (eX) = <MUL (<Eval tE sV':='('"'sH)';'eX>)
                                <Prod tE sV (sL) (<Sub (sH) 1>) (eX)>>;
}
...