code: @Pure
  	public Point2d getPointAt(int groupIndex, int indexInGroup) {
  		final int startIndex = firstInGroup(groupIndex);
  
  		// Besure that the member's index is in the group index's range
  		final int groupMemberCount = getPointCountInGroup(groupIndex);
  		if (indexInGroup < 0) {
  			throw new IndexOutOfBoundsException(indexInGroup + "<0"); //$NON-NLS-1$
  		}
  		if (indexInGroup >= groupMemberCount) {
  			throw new IndexOutOfBoundsException(indexInGroup + ">=" + groupMemberCount); //$NON-NLS-1$
  		}
  
  		return new Point2d(
  				this.pointCoordinates[startIndex + indexInGroup * 2],
  				this.pointCoordinates[startIndex + indexInGroup * 2 + 1]);
  	}
docs: Replies the specified point at the given index in the specified group.
  
  @param groupIndex is the index of the group
  @param indexInGroup is the index of the point in the group (0 for the
  first point of the group...).
  @return the point
  @throws IndexOutOfBoundsException in case of error.