Clarify whether fifth and successive chars in delimited block markers must match fourth char

In Summary of structural containers, the spec shows the initial (typically four) characters of the delimited block (i.e. ==== for an example block).

In the subsequent section Nesting blocks, it says:

When nesting a delimited block that uses the same structural container, it’s necessary to vary the length of the delimiter lines …

It does not say whether the extra characters used to alter the length can be arbitrary or must match the last specified character.

In other words, would ====xyz be valid as an example block delimiter or only ==== plus any number of = characters?

My current working assumption (I'm writing an AsciiDoc parser in pure Rust) is that the extra characters must match the last character specified in the delimiter.