The legend is a list of variables and their domains, usually appearing after a description of the current constraint. This is necessary because the usual visual representation of a constraint contains only the names of the variables in it (see FDBG Annotation), and doesn't show anything about their domain. The legend links these names to the corresponding domains. The legend also shows the changes of the domains made by the constraint. Finally, the legend may contain some conclusions regarding the behavior of the constraint, like failure or side-effects.
The format of the legend is somewhat customizable by defining a hook function; see FDBG Customizing Output. The default format of the legend is the following:
list_2 = 0..3
list_3 = 0..3
list_4 = 0..3
fdvar_2 = 0..3 -> 1..3
Here, we see four variables, with initial domains 0..3, but the
domain of the (previously unnamed) variable fdvar_2 is
narrowed by the constraint (not shown here) to 1..3.
A legend is automatically printed by the built-in visualizer
fdbg_show, but it can be easily printed from user defined
visualizers too.