Path-homotopy is equivalent to $\mathbb{R}$-tree reduction

Abstract: Suppose a path $\alpha$ factors through an $\mathbb{R}$-tree $T$ as $\alpha=q \circ p$. Let $r$ parameterize the unique geodesic in $T$ joining the endpoints of $p$. Then we say that the path $\beta=q \circ r$ is obtained from $\alpha$ by "geodesic $\mathbb{R}$-tree reduction." Essentially, $\beta$ is obtained from $\alpha$ by deleting one-dimensional back-tracking. In this paper, we show that any two homotopic paths are geodesic $\mathbb{R}$-tree reductions of some single common path. Hence, the equivalence relation on paths generated by geodesic $\mathbb{R}$-tree reduction is precisely path-homotopy. The common path is explicitly constructed and is necessarily space-filling in the image of a given path-homotopy.