Infopath concat values in a multi selection list box

in xpath editor enter below value

substring(xdMath:Eval(xdMath:Eval(dfs:dataFields/my:SharePointListItem_RW/my:FieldToConcat/Value[. != ""], 'concat(", ", .)'), ".."), 3)

references
http://sharepoint.stackexchange.com/questions/27154/binding-multi-selection-listbox-in-infopath-form-2010
http://youdhbir.blogspot.co.uk/2012/08/using-multiple-selection-list-box.html