doc: syntax of operators

doc/operator.bnf

+\begin{syntax}
+  op-char-1 ::= "=" | "<" | ">" | "~" ;
+  \
+  op-char-2 ::= "+" | "-";
+  \
+  op-char-3 ::= "*" | "/" | "%" ;
+  \
+  op-char-4 ::= "!" | "$" | "&" | "?" | "@" | "^" | "." | ":" | "|" | "#" ;
+  \
+  op-char ::= op-char-1 | op-char-2 | op-char-3 | op-char-4 ;
+  \
+  infix-op ::= op-char+
+  \
+  prefix-op ::= infix-op | "!" op-char-4* | "?" op-char-4* ; %
+\end{syntax}
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