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digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9";
integer = digit, { digit };
operator = "+" | "-" | "*" | "^";
variable = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z";
polynomial = digit | variable, operator, { polynomial }

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3^x // ein Ausdruck
3^x + 6 - 19 // mehrere Ausdrücke

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3^x 6 + 8 19 - 5 // derzeit gültig bzw. so falsch
3^x + 6 - 19 // mein Ziel bzw. richtig