gUnaryUnion does not work on my SpatialPolygonsDataFrame?

Dear R-sig-Geos,

I am quite new to geographical data in R and also to shape files etc., 
and I have encountered the following difficulty: After reading a 
publicly available shape file on recent EU elections in Berlin (link 
https://www.statistik-berlin-brandenburg.de/opendata/RBS_OD_STI_EU1405.zip)
with the ReadShapePoly command, it contains the Berlin EU election 
"Stimmbezirke" (variable STI), as expected, and they can be plotted.
When using the gUnaryUnion or the gUnionCascaded function from rgeos for 
uniting several "Stimmbezirke" into the larger postal voting units 
(variable BRFW), the result is one single polygon instead of the 
expected 529 such units; plotting it shows a shape that does not explain 
itself to me.

BW_EU <- readShapePoly("RBS_OD_STI_EU1405.shp")  ## shape directory is 
working directory
plot(BW_EU)   ## plots the expected "Stimmbezirke"
BW_EU_BRFW <- gUnionCascaded(BW_EU, "BRFW")  ## gUnaryUnion does not 
make a difference
plot(BW_EU_BRFW)   ## plots only a single outline

I then tried to use gUnaryUnion with the variable STI (which I expected 
to return the unchanged polygons), which apparently returned exactly the 
same outline as the one obtained with BRFW. Presumable, I am overlooking 
something basic here, but I fail to see what. Can someone help?

Thanks in advance!
Best, Ulrike

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## Prof. Ulrike Groemping
## FB II
## Beuth University of Applied Sciences Berlin
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