# File formvalidator.rb, line 570
    def apply_proc_constraint(key, constraint)
      if constraint.call(form[key])
        @form[key] = m[0] if untaint?(key)
      else
        @form.delete(key)
        @invalid_fields[key] ||= []
        unless @invalid_fields.include?(key)
          @invalid_fields[key].push(constraint.inspect)
        end
        nil
      end
    end