Errorbars dor data plots with no X-axis data

Hello,

I am having difficulties figuring out how to add yerrorbars to plot.
Suppose I have following data:

File: a.dat
% Version name  Set1    Set2    Set3    Set1 STDEV  Set2 STDEV  Set3
STDEV
AA-0    20      30      39      1.0     2       3
AA-1    21      32      40      1.1     2       2.5
AB-0    19      30      40      0.6     1.3     2
AB-1    19      29      41      0.6     1.4     0.8
AB-2    22      32      41      1.7     3.0     3.2

There are 3 data sets (Set1-Set3) with ydelta values(columns 5-6).
Since there are no xdata values present, there seems to be no
intuitive way to add error bars using ydelta values from data file.

I use following script to simply plot data sets:

set grid
set key outside
set title "Version historical data"
set style data linespoints
plot \
"a.dat" using 2 title "Set1", \
"a.dat" using 3 title "Set2", \
"a.dat" using 4 title "Set3"

pause -1

How to add yerrorbars to this plot? Is it possible without changing
data file format?

I tired:

plot \
"a.dat" using 2 title "Set1", \
"a.dat" using 3 title "Set2", \
"a.dat" using 4 title "Set3", \
"a.dat" using 2:5 with yerrorbars

Gives an error:
"Not enough columns for this style"

Thanks,
D.