Nested serpentine object S-positions not consistent

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What steps will reproduce the problem?
import beamline
import beamrep
import serpentine as st
from elements import *

bl = beamline.Line()
bl.append(Drift(name='drift0', L=1))
ind = 1
bpmgrd0 = beamline.Line()
bpmgrd1 = beamline.Line()
for d in range(3):
        bpmgrd0.append(BPM(name='bpm'+str(ind),L=0,res=1e-7))
        bpmgrd0.append(Drift(name='drift'+str(ind),L=1))
        ind += 1

for d in range(3):
        bpmgrd1.append(Drift(name='drift'+str(ind),L=1))
        bpmgrd1.append(BPM(name='bpm'+str(ind),L=0,res=1e-7))
        ind += 1

bl.append(st.Serpentine(line=bpmgrd0))
bl.append(Quad(name='clicquad',L=0.5,B=50))
bl.append(st.Serpentine(line=bpmgrd1))

bm = beamrep.Beam(P=11)
s = st.Serpentine(line=bl,beam=bm)

print s.beamline[s.beamline.FindEleByType('Quad')[0]].S
print [b.S for b in s.beamline.GetEleByType('BPM')]

What is the expected output? What do you see instead?
Expected:
4
[1,2,3,5.5,6.5,7.5]

Seen:
1
[1, 2, 3, 2.5, 3.5, 4.5]

What version of the product are you using? On what operating system?
serpentine revision 105
python 2.5.5

Please provide any additional information below.
Thanks

Original issue reported on code.google.com by Francis....@live.rhul.ac.uk on 2 Jun 2011 at 9:16