  The "Phase Response Curve" (PRC) is a common tool used to analyze phase
resetting in the natural sciences. We make the observation that the PRC with
respect to a coordinate $y\in\mathbb{R}$ actually depends on the full choice of
coordinates $(x,y)$, $x\in\mathbb{R}^d$. We give a coordinate-free definition
of the PRC making this observation obvious. We show how by controlling $y$,
using delay coordinates of $y$, and postulating the dynamics of $x$ as a
function of $x$ and $y$, we can sometimes reconstruct the PRC with respect to
the $(x,y)$ coordinates. This suggests a means for obtaining the PRC of, e.g.,
a neuron via a voltage clamp.
