code: public String put(final String key, final String value) {
      return this._current.put(key, value);
    }
docs: Put the given string value for the string key. This method performs any variable substitution
  in the value replacing any occurance of ${name} with the value of get("name").
  
  @param key The key to put the value to
  @param value The value to do substitution on and store
  @throws IllegalArgumentException If the variable given for substitution is not a valid key in
  this Props.