  We investigate the computing power of a restricted class of DNA strand
displacement structures: those that are made of double strands with nicks
(interruptions) in the top strand. To preserve this structural invariant, we
impose restrictions on the single strands they interact with: we consider only
two-domain single strands consisting of one toehold domain and one recognition
domain. We study fork and join signal-processing gates based on these
structures, and we show that these systems are amenable to formalization and to
mechanical verification.
