Syntax:    bf(%negative-dollar-indices) 

    Accept (do not generate warnings) zero- or negative dollar-indices in the
grammar's action blocks. Zero or negative dollar-indices are commonly used to
implement inherited attributes and should normally be avoided. When used they
can be specified like tt($-1), or like tt($<type>-1), where tt(type) is an
tt(STYPE_) tag; or a tt(%union) field-name. See also sections ref(ACTIONS),
ref(SPECIAL), and ref(POLYMORPHIC).
