(defconstant no-bindings '((t . t)))

(defun get-binding (var bindings)
  "Find a (var . value) pair in a binding list"
  (assoc var bindings))

(defun binding-val (bindings)
  "Get the value part of a single binding"
  (cdr binding))

(defun lookup (var bindings)
  "Get the value part (for var) for a binding list"
  (binding-val (get-binding var bindings))

(defun extend-bidings 
  "Add a (var . value) pair to a binding list"
  (cons (cons var val)
        (if (eq bindings no-bindings)
            nil
            bindings)))


##</code><code>##

sub no_bindings
{
	return not keys %{$_[0]};
}


sub is_variable
{
	return substr($_[0], 0, 1) eq '?';
}


sub get_binding
{
	my ($var, $bind) = @_;	
	defined($bind->{$var}) ? [$var, $bind->{$var}] : undef;
}


sub binding_val
{
	defined($_[0]) ? $_[0]->{1} : undef;
}


sub lookup
{
	my ($var, $bind) = @_;
	return $bind->{$var};
}


sub extend_bindings
{
	my ($var, $val, $bind) = @_;
	$bind->{$var} = $val;
	return $bind;
}