class Racc::Grammar

def compute_nullable
  @rules.each       {|r| r.null = false }
  @symboltable.each {|t| t.null = false }
  r = @rules.dup
  s = @symboltable.nonterminals
  begin
    rs = r.size
    ss = s.size
    check_rules_nullable r
    check_symbols_nullable s
  end until rs == r.size and ss == s.size
end