 To be precise, one must somehow find the location of all of the occurrences of the bound variable "V" in the expression "E", implying a time cost, or one must keep track of these locations in some way, implying a space cost.A naïve search for the locations of "V" in "E" is "O"("n") in the length "n" of "E".This has led to the study of systems that use explicit substitution.