_WHITESPACE = " " | "\t" | " ";
_COMMENT    = "(*", {!"*)", <0, 0xFF>}, ("*)" | -);

_           = {_WHITESPACE | _COMMENT};


DIGIT       = <"0", "9">;

ADD         = _, ("+"),             _;
SUB         = _, ("-" | "−"),       _;
MUL         = _, ("*" | "⋅" | "×"), _;
DIV         = _, ("/" | "∕" | "÷"), _;


sign        = ADD | SUB;

_digit      = DIGIT | _WHITESPACE | "_" | "'";
unsigned    = DIGIT, +({_digit}, !_digit);

_number     = unsigned | "(", _expr, (")" | -);

number      = _number, +{_, _number}; (* optionally with implicit multiplication *)

value       = [sign], number;

_expr       = hyper1;


hyper1      = _, hyper2, {+(ADD | SUB), +(hyper2 | -)}, _;
hyper2      = _, value, {+(MUL | DIV), +(value | -)}, _;