Description: Define the set of retractions on two topological spaces. We say that
R is a retraction from J to K . or R e. ( J Retr K ) iff
there is an S such that R : J --> K , S : K --> J are continuous
functions called the retraction and section respectively, and their
composite R o. S is homotopic to the identity map. If a retraction
exists, we say J is a retract of K . (This terminology is
borrowed from HoTT and appears to be nonstandard, although it has
similaries to the concept of retract in the category of topological
spaces and to a deformation retract in general topology.) Two
topological spaces that are retracts of each other are called homotopy
equivalent. (Contributed by Mario Carneiro, 11-Feb-2015)