(15) combine_long_loop sampling step
[6 nts loop from 1S72, OLLM_chain_closure_only]
# COMBINE :
# region_0_3 structure (append-side) :
# N1-N2-G3-A4- -N-..-N47
# AND :
# region_5_0 structure (prepend-side) :
# N1-N2- -G6-A7-A8-N-..-N47
#
# Append-chunk and prepend-chunk tags from tag-pairs found at the
# th line in filtered_tag_pairs.txt
#
# Sample missing nucleotde (U5) and close-chain.