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     Some notes on different structure format versions:     
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* Since version 3 a VERSION entry is present

* Root loop angle:
  * version 1 had none
  * version 2 had one and it pointed to the first segment
  * version 3 has none (angle of root-loop is always handled as absolute angle)

* order of SEGMENTs and STRANDs in LOOPS:
  * normal loops always start and end with a SEGMENT
  * root-loop:
    * version 1+2 ended with a strand
    * version 3 starts with a strand (the fixpoint strand)

* angles
  * version 1 only DELTA
  * version 2 had DELTA and DELTA_IN (loop->strand-angle (rootside) and reverse strand<-loop-angle (outside))
  * version 3 has REL entries in STRAND _and_ LOOP. These angles are relative to angle of father element.