1.1 --- a/doc/lgf.dox	Thu Jul 10 16:05:56 2008 +0200
     1.2 +++ b/doc/lgf.dox	Thu Jul 10 16:13:50 2008 +0200
     1.3 @@ -78,10 +78,18 @@
     1.4   2   3   18
     1.5  \endcode
     1.6  
     1.7 -The \c \@edges is just a synonym of \c \@arcs.
     1.8 +The \c \@edges is just a synonym of \c \@arcs. The @arcs section can
     1.9 +also store the edge set of an undirected graph. In such case there is
    1.10 +a conventional method for store arc maps in the file, if two columns
    1.11 +has the same caption with \c '+' and \c '-' prefix, then these columns
    1.12 +can be regarded as the values of an arc map.
    1.13  
    1.14  The \c \@attributes section contains key-value pairs, each line
    1.15 -consists of two tokens, an attribute name, and then an attribute value.
    1.16 +consists of two tokens, an attribute name, and then an attribute
    1.17 +value. The value of the attribute could be also a label value of a
    1.18 +node or an edge, or even an edge label prefixed with \c '+' or \c '-',
    1.19 +which regards to the forward or backward directed arc of the
    1.20 +corresponding edge.
    1.21  
    1.22  \code
    1.23   @attributes