E: Expression
   N: Number
   D: Digit

   E ::= E1 '+' E2 | E1 '–' E2 | E1 '*' E2 | '(' E ')' | N
   N ::= N D | D
   D ::= '0' | '1' | ... | '9'

    Domain v: Integer = { ..., -2, -1, 0, 1, 2, ...}
    Operations
    +: Integer × Integer → Integer
    –: Integer × Integer → Integer
    *: Integer × Integer → Integer

   E: Expression → Integer
   E[[E1 '+' E2]] = E[[E1]] + E[[E2]]
   E[[E1 '–' E2]] = E[[E1]] – E[[E2]]
   E[[E1 '*' E2]] = E[[E1]] * E[[E2]]
   E[['(' E ')']] = E[[E]]
   E[[N]] = N[[N]]
   N: Number → Integer 
   N[[ND]] = 10 * N[[N]] + N[[D]]
   N[[D]]  = D[[D]]
   D: Digit → Integer
   D[['0']] = 0, D[['1']] = 1, ..., D[['9']] = 9