#' Single First-Order kinetics
#'
#' Function describing exponential decline from a defined starting value.
#'
#' @param t Time.
#' @param parent.0 Starting value for the response variable at time zero.
#' @param k Kinetic constant.
#' @return The value of the response variable at time \code{t}.
#' @references FOCUS (2006) \dQuote{Guidance Document on Estimating Persistence
#'   and Degradation Kinetics from Environmental Fate Studies on Pesticides in
#'   EU Registration} Report of the FOCUS Work Group on Degradation Kinetics,
#'   EC Document Reference Sanco/10058/2005 version 2.0, 434 pp,
#'   \url{http://esdac.jrc.ec.europa.eu/projects/degradation-kinetics}
#' @examples
#'
#'   \dontrun{plot(function(x) SFO.solution(x, 100, 3), 0, 2)}
#'
#' @export
SFO.solution <- function(t, parent.0, k)
{
	parent = parent.0 * exp(-k * t)
}