add identical attribute to EBML Element Definitions

specification.markdown

@@ -193,6 +193,7 @@ Within an EBML Schema the `<element>` uses the following attributes to define an
 | id             | Yes      | The Element ID expressed in hexadecimal notation prefixed by a '0x'. |
 | mandatory      | No       | A boolean to express if the EBML Element MUST occur if the Parent EBML Element is used. If the mandatory attribute is not expressed for that Element then that element is to be considered not mandatory. |
 | multiple       | No       | A boolean to express if the EBML Element may occur within its Parent EBML Element more than once. If the multiple attribute is false or the multiple attribute is not used to define the Element then that EBML Element MUST not occur more than once with that Element's Parent EBML Element. |
+| identical      | No       | A boolean to express if the EBML Element may occur within its Parent Element more than once but that each recurrence within that Parent Element MUST be identical both in storage and semantics. Such Elements are referred to as Identically-Recurring Elements. In this case, identical copies of an EBML Element are permitted to be stored multiple times within the same Parent Element in order to increase data resilience and optimize the use of EBML in transmission. Identically-Recurring Elements SHOULD include a CRC-32 Element as a Child Element; this is especially recommended when EBML is used for long-term storage or transmission. If a Parent Element contains more than one copy of an Identically-Recurring Element which includes a CRC-32 Child Element then the first instance of the Identically-Recurring Element with a valid CRC-32 value should be used for interpretation. If a Parent Element contains more than one copy of an Identically-Recurring Element which does not contain a CRC-32 Child Element or if CRC-32 Child Elements are present but none are valid then the first instance of the Identically-Recurring Element should be used for interpretation. If the `identical` attribute is not expressed for that Element then that Element is considered to not have a requirement for identical expression within the same Parent Element. The `identical` attribute is only valid if the Element is not set to `multiple`, otherwise the `identical` attribute shall be ignored. |
 | range          | No       | For Elements which are of numerical types (Unsigned Integer, Signed Integer, Float, and Date) a numerical range may be specified. If specified that the value of the EBML Element MUST be within the defined range inclusively. See the [section of Expressions of range](#expression-of-range) for rules applied to expression of range values. |
 | default        | No       | A default value may be provided. If an Element is mandatory but not written within its Parent EBML Element, then the parser of the EBML Document MUST insert the defined default value of the Element. EBML Elements that are Master-elements MUST NOT declare a default value. |
 | type           | Yes      | As defined within the [section on EBML Element Types](#ebml-element-types), the type MUST be set to one of the following values: 'integer' (signed integer), 'uinteger' (unsigned integer), 'float', 'string', 'date', 'utf-8', 'master', or 'binary'. |