Elisp dynamically varying scoping

Emacs 24 introduced lexical scope for variables to Emacs
Lisp.  However, rather than provide it via a new form, say llet or
whatever, the buffer local variable `lexical-binding' was
introduced to switch the behaviour of binding forms.  This is an
unfortunate situation because the semantics of a piece of code
depend on the value of a buffer local variable at the time of
evaluation.