$ENTRY Go { 
    = <Prout <Calc <Spar <Check<Card>>>>>; 
    }

Check {
  e.1 = <DoCheck ('TF&|<=>()!') e.1>;
}    
    
DoCheck {
    (e.L s.X e.R) s.X e.1 = 
        s.X<DoCheck (e.L s.X e.R) e.1>;
    (e.S) s.X e.1 = <DoCheck (e.S) e.1>;
    (e.S) = ;
}
    
Spar { e.1 = <Spar1 (L) e.1>; }

Spar1 {
    (e.S) '(' e.T = <Spar1 ((e.S)) e.T>;
    ((e.S) e.1) ')' e.T = 
        <Spar1 (e.S (e.1)) e.T>;
    (L e.S) ')' e.T = ERROR;
    (e.S) s.X e.T = 
        <Spar1 (e.S s.X) e.T>;
    (L e.S) = e.S;
    (e.S) = ERROR;
}    

Calc {
    '!' e.1 = <Not <Calc e.1>>;
    e.1 '<=>' e.2 = <Equivalence <Calc e.1> <Calc e.2>>;
    e.1 '=>' e.2 = <Implication <Calc e.1> <Calc e.2>>;
    e.1 '|' e.2 = <Or <Calc e.1> <Calc e.2>>;
    e.1 '&' e.2 = <And <Calc e.1> <Calc e.2>>;
    ( e.1 )     = <Calc e.1>;
    s.1         = s.1;
}
 
Equivalence {
  s.1 s.1 = 'T';
  e.1 = 'F';
} 

Implication {
  F s.1 = 'T';
  s.1 s.1 = 'T';
  e.1 = 'F';
}
 
Or {
  'F' 'F' = 'F';
  e.1 = 'T';
} 

And {
  'T' 'T' = 'T';
  e.1 = 'F';
}

Not {
  'T' = 'F';
  'F' = 'T';

}