plyr's merge: Join by any other name.
Sample AST: (merge $leftFrame $rightFrame allLeftFlag allRightFlag)
Joins two frames; all columns with the same names will be the join key. If
you want to join on a subset of identical names, rename the columns first
(otherwise the same column name would appear twice in the result).
If allLeftFlag is true, all rows in the leftFrame will be included, even if
there is no matching row in the rightFrame, and vice-versa for
allRightFlag. Missing data will appear as NAs. Both flags can be true.