def require_program(*keys)
      keys.each do |key|
        fail CommandError, "program #{key} required" if program(key).nil? || program(key).empty?
      end
    end
def require_program(*keys)
      keys.each do |key|
        fail CommandError, "program #{key} required" if program(key).nil? || program(key).empty?
      end
    end
Raises a CommandError when the program any of the _keys_ are not present, or empty.