Figure out margining variables.
Given the variables that form the rows and columns, and a set of desired margins, works out which ones are possible. Variables that can't be margined over are dropped silently.
margins(vars, margins = NULL)
a list of character vectors giving the variables in each dimension
a character vector of variable names to compute margins for.
TRUEwill compute all possible margins.
list of margining combinations, or
NULL if none. These are
the combinations of variables that should have their values set to