def _pv_default(opts, key, default_value)
        value = _pv_opts_lookup(opts, key)
        if value == nil
          opts[key] = default_value
        end
      end
def _pv_default(opts, key, default_value)
        value = _pv_opts_lookup(opts, key)
        if value == nil
          opts[key] = default_value
        end
      end
Assign a default value to a parameter.