Is "unique names" enough?

Environments.Rmd

@@ -59,10 +59,12 @@ The `env_` functions in rlang are designed to work with the pipe: all take an en
 
 ## Environment basics {#env-basics}
 
-Generally, an environment is similar to a named list, with four important exceptions:
+Generally, an environment is similar to a named list, with five important exceptions:
 
 *   Every name must be unique.
 
+*   All elements must be binded to a name.
+
 *   The names in an environment are not ordered.
 
 *   An environment has a parent. 
@@ -896,7 +898,7 @@ As well as powering scoping, environments are also useful data structures in the
 
 ## Quiz answers {#env-answers}
 
-1.  There are four ways: every object in an environment must have a name;
+1.  There are five ways: every name must be unique; every element in an environment must be binded to a name;
     order doesn't matter; environments have parents; environments have
     reference semantics.