  Gene assembly in ciliates is an extremely involved DNA transformation
process, which transforms a nucleus, the micronucleus, to another functionally
different nucleus, the macronucleus. In this paper we characterize which loop
recombination operations (one of the three types of molecular operations that
accomplish gene assembly) can possibly be applied in the transformation of a
given gene from its micronuclear form to its macronuclear form. We also
characterize in which order these loop recombination operations are applicable.
This is done in the abstract and more general setting of so-called legal
strings.
