expand combinations of variables:values to columnar format
macros.xml
@Article{,
title = {Reshaping Data with the {reshape} Package},
author = {Hadley Wickham},
journal = {Journal of Statistical Software},
year = {2007},
volume = {21},
number = {12},
pages = {1--20},
url = {http://www.jstatsoft.org/v21/i12/},
}