#### 3.3.2 BNF ¶

The following specification in Backus-Naur form describes formally the Mailutils debug level:

    <debug-spec> ::= <level-spec> | <debug-level-list>
    <debug-level-list> ::= <debug-level> |
                           <debug-level-list> ";" <debug-level>
    <debug-level> ::= <category> | "!" <category> |
                      <category> "." <level-list>
    <level-list> ::= <level-spec> | <level-list> "," <level-spec>
    <level-spec> ::=  <level> | <negate-level>
    <negate-level> ::= "!" <level>
    <level> ::= <level-number> | "=" <level-number> |
                <level-number> "-" <level-number>
    <level-number> ::= "error" | "trace0" | "trace1" | "trace2" | "trace3" |
                       "trace4" | "trace5" | "trace6" | "trace7" |
                       "trace8" | "trace9" | "prot"