  We introduce a variant of modal logic, dubbed EXISTENTIAL COUNTING MODAL
LOGIC (ECML), which captures a vast majority of problems known to be tractable
in single exponential time when parameterized by treewidth. It appears that all
these results can be subsumed by the theorem that model checking of ECML admits
an algorithm with such complexity. We extend ECML by adding connectivity
requirements and, using the Cut&Count technique introduced by Cygan et al. [4],
prove that problems expressible in the extension are also tractable in single
exponential time when parameterized by treewidth; however, using randomization.
The need for navigationality of the introduced logic is justified by a negative
result that two expository problems involving non-acyclic conditions, C_l
VERTEX DELETION and GIRTH>l VERTEX DELETION for l>=5, do not admit such a
robust algorithm unless Exponential Time Hypothesis fails.
