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RFC: Unicode support

文法:

Arithmetic:
    Term     < Factor (Add / Sub)*
    Add      < "+" Factor
    Sub      < "-" Factor
    Factor   < Primary (Mul / Div)*
    Mul      < "*" Primary
    Div      < "/" Primary
    Primary  < Parens / Neg / Pos / Number / Variable
    Parens   < "(" Term ")"
    Neg      < "-" Primary
    Pos      < "+" Primary
    Number   < ~([0-9]+)

    Variable <- identifier

数式
1 + 2 - (3*x-5)*6

生成結果:

Arithmetic("1 + 2 - (3*x-5)*6").writeln;
=>
    Arithmetic [0, 17]["1", "+", "2", "-", "(", "3", "*", "x", "-", "5", ")", "*", "6"]
    +-Arithmetic.Term [0, 17]["1", "+", "2", "-", "(", "3", "*", "x", "-", "5", ")", "*", "6"]
        +-Arithmetic.Factor [0, 2]["1"]
        |  +-Arithmetic.Primary [0, 2]["1"]
        |     +-Arithmetic.Number [0, 2]["1"]
        +-Arithmetic.Add [2, 6]["+", "2"]
        |  +-Arithmetic.Factor [4, 6]["2"]
        |     +-Arithmetic.Primary [4, 6]["2"]
        |        +-Arithmetic.Number [4, 6]["2"]
        +-Arithmetic.Sub [6, 17]["-", "(", "3", "*", "x", "-", "5", ")", "*", "6"]
        +-Arithmetic.Factor [8, 17]["(", "3", "*", "x", "-", "5", ")", "*", "6"]
            +-Arithmetic.Primary [8, 15]["(", "3", "*", "x", "-", "5", ")"]
            |  +-Arithmetic.Parens [8, 15]["(", "3", "*", "x", "-", "5", ")"]
            |     +-Arithmetic.Term [9, 14]["3", "*", "x", "-", "5"]
            |        +-Arithmetic.Factor [9, 12]["3", "*", "x"]
            |        |  +-Arithmetic.Primary [9, 10]["3"]
            |        |  |  +-Arithmetic.Number [9, 10]["3"]
            |        |  +-Arithmetic.Mul [10, 12]["*", "x"]
            |        |     +-Arithmetic.Primary [11, 12]["x"]
            |        |        +-Arithmetic.Variable [11, 12]["x"]
            |        +-Arithmetic.Sub [12, 14]["-", "5"]
            |           +-Arithmetic.Factor [13, 14]["5"]
            |              +-Arithmetic.Primary [13, 14]["5"]
            |                 +-Arithmetic.Number [13, 14]["5"]
            +-Arithmetic.Mul [15, 17]["*", "6"]
                +-Arithmetic.Primary [16, 17]["6"]
                    +-Arithmetic.Number [16, 17]["6"]