meaning: Program → Integer

   meaning[[P]] = d where perform[[P]]( 0, nop, 0, 0 ) = ( a, op, d, m )


   perform: ExprSequence → ( State → State )

   perform[[E S]] = perform[[S]] º evaluate[[E]]
   perform[[E]] = evaluate[[E]]


   evaluate: Expression → ( State → State )

   evaluate[[N]]( a, op, d, m) = ( a, op, v, m ) where v = value[[N]]
   evaluate[[MR]]( a, op, d, m ) = ( a, op, m, m )
   evaluate[[Clear]]( a, op, d, m ) = ( 0, nop, 0, 0 )
   evaluate[[E1 O E2]] = evaluate[[E2]] º compute[[O]] º evaluate[[E1]]
   evaluate[[E A]] = calculate[[A]] º evaluate[[E]]


   compute: Operator → ( State → State )

   compute[[+]]( a, op, d, m ) = ( op( a, d ), plus, op( a, d ), m )
   compute[[–]]( a, op, d, m ) = ( op( a, d ), minus, op( a, d ), m )
   compute[[×]]( a, op, d, m ) = ( op( a, d ), times, op( a, d ), m )


   calculate: Answer → ( State → State )

   calculate[[=]]( a, op, d, m ) = ( a, nop, op( a, d ), m )
   calculate[[M+]]( a, op, d, m ) = ( a, nop, v, plus( m, v ) )
                                      where v = op( a, d )
   calculate[[+/-]]( a, op, d, m ) = ( a, op, minus( 0, d ), m )


   value: Numeral → Integer

   value[[ND]] = 10 * value[[N]] + value[[D]]
   value[[D]]  = digit[[D]]


   digit: Digit → Integer

   digit[['0']] = 0, digit[['1']] = 1, ..., digit[['9']] = 9