expectation of product

[affeldt-aist]

Motivation for this change

Checklist

• [x] added corresponding entries in CHANGELOG_UNRELEASED.md

• [x] added corresponding documentation in the headers

Reference: How to document

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
• Check labels

[affeldt-aist]

Maybe it is better to rename expectation_prod to expectation_mul since the original intent was expectationM.