A Constraint is used to control the values in a Binding.
This is a very general mechanism, since you can change the Constaint
associated with a Binding as needed.
Get value of "function binding" of a Binding.
Some languages (including Common Lisp and Emacs Lisp) associate both
a value binding and a function binding with a symbol.
the function value, or Binding.UNBOUND if no function binding.