let symbol_of s = List.assoc s
[
",", COMMA; ";", SEMICOLON; ":", COLON; ".", DOT;
"{", LBRACE; "}", RBRACE; "[", LBRACKET; "]", RBRACKET;
"(", LPAREN; ")", RPAREN;
"=", EQUAL; "?", QUESTION; "|", VBAR; "^", HAT; "&", AMPERSAND;
"<", LANGLE; ">", RANGLE;
"+", PLUS; "-", MINUS; "*", STAR; "/", SLASH; "%", PERCENT;
"~", TILDE; "!", EXCL
]