Before revision:  We model the firm's optimal procurement problem as a singular stochastic control problem in which a nondecreasing control policy represents the cumulative investment made by the firm in the spot market ( that is, a so-called stochastic "monotone follower problem").
After revision:  We model the firm's optimal procurement problem as a singular stochastic control problem in which controls are nondecreasing processes and represent the cumulative investment made by the firm in the spot market ( that is, a so-called stochastic "monotone follower problem").