# File lib/ruote/exp/fe_inc.rb, line 124
    def apply

      if var_key = has_attribute(:v, :var, :variable)

        var = attribute(var_key)
        set_v(var, new_value(:var, var))

      elsif field_key = has_attribute(:f, :fld, :field)

        field = attribute(field_key)
        set_f(field, new_value(:field, field))

      else

        k = attribute_text

        raise(
          ArgumentError.new('no variable or field to increment/decrement')
        ) if k.length < 1

        set_vf(k, new_value(nil, k))

      #else
      #  raise ArgumentError.new(
      #    "missing a variable or field target in #{tree.inspect}")
      end

      reply_to_parent(h.applied_workitem)
    end