Is there a option to change the seperator for multi-header from dot to underscore ?

### Question
When a parent header is merged with child header , dot is used as separator like "parentheader.childheader" , is it possible to use underscore or any user-defined separator ?
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Start by locating the multi-header handling that joins parent and child headers with a dot, then check nearby tests or examples for the current behavior. Done means supporting underscore or another user-defined separator without breaking the existing dot-separated output.

Written by the indexing model from the issue text.