message: trace-iterator to work with unit subsets
diff: @@ -222,9 +222,9 @@ export class AtomicPolymerTraceIterator implements Iterator<PolymerTraceElement>
  const residueIndexNext2 = this.getResidueIndex(residueIndex + 2)
  const residueIndexNext3 = this.getResidueIndex(residueIndex + 3)
  - if (value.first) {
  value.centerPrev.element = this.traceElementIndex[residueIndexPrev1]
  value.center.element = this.traceElementIndex[residueIndex]
  + value.centerNext.element = this.traceElementIndex[residueIndexNext1]
  this.pos(this.p0, this.traceElementIndex[residueIndexPrev3])
  this.pos(this.p1, this.traceElementIndex[residueIndexPrev2])
  @@ -232,27 +232,11 @@ export class AtomicPolymerTraceIterator implements Iterator<PolymerTraceElement>
  this.pos(this.p3, this.traceElementIndex[residueIndex])
  this.pos(this.p4, this.traceElementIndex[residueIndexNext1])
  this.pos(this.p5, this.traceElementIndex[residueIndexNext2])
  + this.pos(this.p6, this.traceElementIndex[residueIndexNext3])
  this.setFromToVector(this.d01, residueIndexPrev1)
  this.setFromToVector(this.d12, residueIndex)
  this.setFromToVector(this.d23, residueIndexNext1)
  - } else {
  - value.centerPrev.element = value.center.element
  - value.center.element = value.centerNext.element
  -
  - Vec3.copy(this.p0, this.p1)
  - Vec3.copy(this.p1, this.p2)
  - Vec3.copy(this.p2, this.p3)
  - Vec3.copy(this.p3, this.p4)
  - Vec3.copy(this.p4, this.p5)
  - Vec3.copy(this.p5, this.p6)
  -
  - Vec3.copy(this.d01, this.d12)
  - Vec3.copy(this.d12, this.d23)
  - Vec3.copy(this.d23, this.d34)
  - }
  - value.centerNext.element = this.traceElementIndex[residueIndexNext1]
  - this.pos(this.p6, this.traceElementIndex[residueIndexNext3])
  this.setFromToVector(this.d34, residueIndexNext2)
  this.setControlPoint(value.p0, this.p0, this.p1, this.p2, residueIndexPrev2)
  