I would a strategy for adding of GU-pairs that says, for adding of a GU-pair, a string should be 4 nucleotides long or longer.
Give -1 if a GU-pair is placed in a string 4 nt’s long and with tetraloop
Give -1 if a GU-pair is placed in a string 3 n’ts long
Give -2 if a GU-pair is placed in a string 2 nt’s long.
Also the string must have no tetraloop. If a string is shorter than 4 nt’s long and have a tetraloop, give -1 extra for the tetraloop.