Retrieves the value according to the currently visible binding.
Adds a binding to the innermost scope.
Bindings (and what they shadow).
Checks whether a key is bound in the innermost scope.
Checks whether a key is bound at all.
Enters a nested scope.
Leaves a nested scope.
Which keys are bound in all the scopes.
Updates the currently visible binding.
Mutable symbol table with support for nested scopes.