fix #279, update description

einops/_backends.py

@@ -3,11 +3,11 @@
 - backends are not imported unless those are actually needed, because
  - backends may not be installed
  - importing all available backends will drive to significant memory footprint
- - backends may by present but installed with errors (but never used),
+ - backends may be present but installed with errors (but never used),
  importing may drive to crashes
-- backend should be either symbolic or imperative (tensorflow is for both, but that causes problems)
+- backend should be either symbolic or imperative
  - this determines which methods (from_numpy/to_numpy or create_symbol/eval_symbol) should be defined
-- if backend can't (temporarily) provide symbols for shape dimensions, UnknownSize objects are used
+- if backend can't provide symbols for shape dimensions, UnknownSize objects are used
 """
 
 import sys
@@ -29,7 +29,7 @@ def get_backend(tensor) -> "AbstractBackend":
  if _result is not None:
  return _result
 
- for framework_name, backend in _loaded_backends.items():
+ for framework_name, backend in list(_loaded_backends.items()):
  if backend.is_appropriate_type(tensor):
  _type2backend[_type] = backend
  return backend