[R] Contatenating data frames with partial overlap in variable names

[Daniel Folkinshteyn]

Greetings to all.
I need to concatenate data frames that do not have all the same variable
names, there is only a partial overlap in the variables. So, for
example, if i have two data frames, a and b, that look like the following:
> a
  a b
1 1 4
2 2 5
3 3 6
4 4 7
5 5 8
> b
  c  a
1 1 10
2 2 11
3 3 12
4 4 13
5 5 14

i want to concatenate them by row, without any matching, so that the
variables that are not available in all frames get NAs. The result
should look like:

   a  b  c
1  1  4  NA
2  2  5  NA
3  3  6  NA
4  4  7  NA
5  5  8  NA
6  10 NA 1
7  11 NA 2
8  12 NA 3
9  13 NA 4
10 14 NA 5

rbind doesn't work, since it requires all variables to be matched
between the two data frames. merge doesn't work, since it wants to
/match/ by columns with the same name, and if matching by nothing,
produces a cartesian product.

is there a neat trick for doing this simply, or am i stuck with
comparing variable lists and generating NAs manually?

would appreciate any help!
Daniel