can configure
the attribute it listens to with the `attr-for-value` attribute.

### Using a custom input element

You can use a custom input element in a `<paper-input-container>`, for example to implement a
compound input field like a social security number input. The custom input element should have the
`paper-input-input` class, have a `notify:true` value property and optionally implements
`Polymer.IronValidatableBehavior` if it is validatable.

    <paper-input-container attr-for-value="ssn-value">
    