public String execute(SampleResult previousResult,
    Sampler currentSampler) throws InvalidVariableException
Given the previous SampleResult and the current Sampler, return a string
to use as a replacement value for the function call. Assume
"setParameter" was previously called.

This method must be threadsafe - multiple threads will be using the same
object.

 public List getArgumentDesc()
Return a list of strings briefly describing each parameter your function
takes. Please use JMeterUtils.getResString(resource_name) to grab a
resource string. Otherwise, your help text will be difficult to
internationalize.

This list is not optional. If you don't wish to write help, you must at
least return a List containing the correct number of blank strings, one
for each argument.

 public String getReferenceKey()
Return the name of your function. Convention is to prepend "__" to the
name (ie "__regexFunction")

 public  void setParameters(Collection parameters) throws InvalidVariableException
A collection of the parameters used to configure your function. Each
parameter is a CompoundFunction and can be resolved by calling the
execute() method of the CompoundFunction (which should be done at
execution.)