# File lib/ruote/exp/ro_variables.rb, line 107
    def unset_variable (var)

      fexp, v = locate_var(var)

      raise(
        ArgumentError.new("cannot set var at engine level : #{var}")
      ) if fexp.nil?

      should_persist = (fexp.h.fei != h.fei)
        # don't use a ticket when expression wants to modify its own vars

      fexp.un_set_variable(:unset, v, nil, should_persist)
    end