@classmethod
def envkey(cls, key, namespace=None):
    """ Transform a mapping key, along with an optional namespace
            value and the classes’ “appname” value, into an environment-
            variable name. Like e.g., for an appname of “YoDogg” and
            a namespace value of “iheard”, the environment variable
            prefix would work out such that a variable with a key value
            of “youlike” would look like this:
                
                YODOGG_IHEARD_YOULIKE
                                     
                ^^^^^^ ^^^^^^ ^^^^^^^
                   |      |      |
                   |      |      +––––– mapping key (uppercased)
                   |      +–––––––––––– namespace value (uppercased)
                   +––––––––––––––––––– app name (uppercased)
        """
    return cls.prefix(namespace=namespace) + str(key).upper()
