code: def from_untrusted(cls, value, lineno=None, environment=None):
          
          from .compiler import has_safe_repr
          if not has_safe_repr(value):
              raise Impossible()
          return cls(value, lineno=lineno, environment=environment)
docs: Return a const object if the value is representable as
          constant value in the generated code, otherwise it will raise
          an `Impossible` exception.