# File formvalidator.rb, line 624
    def apply_proc_constraint(key, constraint)
      if res = constraint.call(@form[key])
        if untaint?(key)
          @form[key] = res 
          @form[key].untaint
        end
      else
        @form.delete(key)
        @invalid_fields[key] ||= []
        unless @invalid_fields[key].include?(constraint.inspect)
          @invalid_fields[key].push(constraint.inspect)
        end
        nil
      end
    end