code: def _check_environ(variable, value):
      
      if is_not_none(value):
          return value
      else:
          value = os.environ.get(variable)
          if is_none(value):
              stop(''.join([variable,
                            """ not supplied and no entry in environmental
                             variables"""]))
          else:
              return value
docs: check if a variable is present in the environmental variables