  A two-dimensional simplicial complex is called $d$-{\em regular} if every
edge of it is contained in exactly $d$ distinct triangles. It is called
$\epsilon$-expanding if its up-down two-dimensional random walk has a
normalized maximal eigenvalue which is at most $1-\epsilon$.
  In this work, we present a class of bounded degree 2-dimensional expanders,
which is the result of a small 2-complex action on a vertex set. The resulted
complexes are fully transitive, meaning the automorphism group acts
transitively on their faces.
  Such two-dimensional expanders are rare! Known constructions of such bounded
degree two-dimensional expander families are obtained from deep algebraic
reasonings (e.g. coset geometries).
  We show that given a small $d$-regular two-dimensional $\epsilon$-expander,
there exists an $\epsilon'=\epsilon'(\epsilon)$ and a family of bounded degree
two-dimensional simplicial complexes with a number of vertices goes to
infinity, such that each complex in the family satisfies the following
properties:
  * It is $4d$-regular.
  * The link of each vertex in the complex is the same regular graph (up to
isomorphism).
  * It is $\epsilon'$ expanding.
  * It is transitive.
  The family of expanders that we get is explicit if the one-skeleton of the
small complex is a complete multipartite graph, and it is random in the case of
(almost) general $d$-regular complex. For the randomized construction, we use
results on expanding generators in a product of simple Lie groups. This
construction is inspired by ideas that occur in the zig-zag product for graphs.
It can be seen as a loose two-dimensional analog of the replacement product.
