1.1 --- a/doc/lgf.dox	Wed Jul 13 14:52:22 2011 +0200
     1.2 +++ b/doc/lgf.dox	Thu Aug 04 21:33:17 2011 +0200
     1.3 @@ -63,11 +63,11 @@
     1.4   3      (40,10)      10      "Third node"
     1.5  \endcode
     1.6  
     1.7 -The \c \@arcs section is very similar to the \c \@nodes section,
     1.8 -it again starts with a header line describing the names of the maps,
     1.9 -but the \c "label" map is not obligatory here. The following lines
    1.10 -describe the arcs. The first two tokens of each line are
    1.11 -the source and the target node of the arc, respectively, then come the map
    1.12 +The \c \@arcs section is very similar to the \c \@nodes section, it
    1.13 +again starts with a header line describing the names of the maps, but
    1.14 +the \c "label" map is not obligatory here. The following lines
    1.15 +describe the arcs. The first two tokens of each line are the source
    1.16 +and the target node of the arc, respectively, then come the map
    1.17  values. The source and target tokens must be node labels.
    1.18  
    1.19  \code
    1.20 @@ -78,10 +78,21 @@
    1.21   2   3   18
    1.22  \endcode
    1.23  
    1.24 +If there is no map in the \c \@arcs section at all, then it must be
    1.25 +indicated by a sole '-' sign in the first line.
    1.26 +
    1.27 +\code
    1.28 + @arcs
    1.29 +         -
    1.30 + 1   2
    1.31 + 1   3
    1.32 + 2   3
    1.33 +\endcode
    1.34 +
    1.35  The \c \@edges is just a synonym of \c \@arcs. The \@arcs section can
    1.36  also store the edge set of an undirected graph. In such case there is
    1.37  a conventional method for store arc maps in the file, if two columns
    1.38 -has the same caption with \c '+' and \c '-' prefix, then these columns
    1.39 +have the same caption with \c '+' and \c '-' prefix, then these columns
    1.40  can be regarded as the values of an arc map.
    1.41  
    1.42  The \c \@attributes section contains key-value pairs, each line