let symbol_of_string = fun s ->
                        match s with
                            |"+"                        ->        Add
                        |"-"                        ->        Sup                
                        |"*"                        ->        Mul
                        |"/"                        ->        Div
                        |"_"                        ->        Pass
                        |"!"                        ->        Stop
                        |"mem"                        ->        Mem
                        |"@"                        ->        Delay
                        |"floor"                ->        Floor
                        |"int"                        ->        Int
                        |"sin"                        ->        Sin
                        |"cos"                  ->      Cos
                        |"atan"                 ->      Atan
                        |"atantwo"              ->      Atantwo
                        |"sqrt"                 ->      Sqrt
                        |"rdtable"                ->        Rdtable
                        |"%"                        ->        Mod
                        |"vectorize"                ->        Vectorize
                        |"#"                        ->        Concat
                        |"[]"                        ->        Nth
                        |"serialize"                ->        Serialize
                        |">"                        ->        Larger
                        |"<"                        ->        Smaller
                        |"prefix"                ->        Prefix
                        |"selecttwo"                ->        Selecttwo
                        |"selectthree"                ->        Selectthree
                        | _                     ->      raise Symbol_not_defined