 if the version of {TAIL ~ the current node} corresponds to the stored map version at {HEAD ~ the step} , matching node coordinates with the stored map and extracting corresponding nodes; and 
if the version of { TAIL ~ the current node } corresponds to the stored map version at { HEAD ~ the step } , matching node coordinates with the stored map and extracting corresponding nodes ; and
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH