1. A method for modeling a variable of an element in a Hardware Description Language (HDL) subprogram of a HDL program, the method comprising: providing the HDL subprogram with an initial value of the variable; storing the HDL subprogram containing the initial value of the variable; in response to a call to the HDL subprogram, executing, by a computer processor, the HDL subprogram by: providing to the HDL program a copy of the stored HDL subprogram comprising a copy of the variable, wherein the value of the copy of the variable is the initial value, and modifying, in the copy of the HDL subprogram, the value of the copy of the variable from the initial value to a second value, while the value of the variable in the stored HDL subprogram remains the initial value, wherein the initial value of the variable is retained in the stored HDL subprogram during and after execution of the copy of the stored HDL subprogram.